diff --git a/.rubocop.yml b/.rubocop.yml
index 40337cf..9302fb3 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -13,6 +13,12 @@ Metrics/MethodLength:
   CountComments: false
   Max: 15
 
+Metrics/CyclomaticComplexity:
+  Max: 8
+
+Metrics/PerceivedComplexity:
+  Max: 8
+
 ## Styles ######################################################################
 
 Style/AlignParameters:
diff --git a/lib/http/form_data/composite_io.rb b/lib/http/form_data/composite_io.rb
new file mode 100644
index 0000000..dbcc5ee
--- /dev/null
+++ b/lib/http/form_data/composite_io.rb
@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+require "stringio"
+
+module HTTP
+  module FormData
+    # Provides IO interface across multiple IO objects.
+    class CompositeIO
+      # @param [Array<IO>] ios Array of IO objects
+      def initialize(ios)
+        ios = ios.map do |io|
+          if io.is_a?(String)
+            StringIO.new(io)
+          elsif io.respond_to?(:read)
+            io
+          else
+            fail ArgumentError, "#{io.inspect} is neither a String nor an IO object"
+          end
+        end
+
+        @ios = ios
+        @index = 0
+        @buffer = String.new
+      end
+
+      # Reads and returns partial content acrosss multiple IO objects.
+      #
+      # @param [Integer] length Number of bytes to retrieve
+      # @param [String] outbuf String to be replaced with retrieved data
+      #
+      # @return [String, nil]
+      def read(length = nil, outbuf = nil)
+        outbuf = outbuf.to_s.replace("")
+
+        while current_io
+          current_io.read(length, @buffer)
+          outbuf << @buffer
+          length -= @buffer.length if length
+
+          break if length && length.zero?
+
+          advance_io
+        end
+
+        outbuf unless length && outbuf.empty?
+      end
+
+      # Returns sum of all IO sizes.
+      def size
+        @size ||= @ios.map(&:size).inject(0, :+)
+      end
+
+      # Rewinds all IO objects and set cursor to the first IO object.
+      def rewind
+        @ios.each(&:rewind)
+        @index = 0
+      end
+
+      private
+
+      # Returns IO object under the cursor.
+      def current_io
+        @ios[@index]
+      end
+
+      # Advances cursor to the next IO object.
+      def advance_io
+        @index += 1
+      end
+    end
+  end
+end
diff --git a/lib/http/form_data/file.rb b/lib/http/form_data/file.rb
index a39bee7..85cfc8e 100644
--- a/lib/http/form_data/file.rb
+++ b/lib/http/form_data/file.rb
@@ -26,15 +26,14 @@ class File < Part
       alias mime_type content_type
 
       # @see DEFAULT_MIME
-      # @param [String, StringIO, File] file_or_io Filename or IO instance.
+      # @param [String, Pathname, IO] path_or_io Filename or IO instance.
       # @param [#to_h] opts
       # @option opts [#to_s] :content_type (DEFAULT_MIME)
       #   Value of Content-Type header
       # @option opts [#to_s] :filename
-      #   When `file` is a String, defaults to basename of `file`.
-      #   When `file` is a File, defaults to basename of `file`.
-      #   When `file` is a StringIO, defaults to `"stream-{object_id}"`
-      def initialize(file_or_io, opts = {})
+      #   When `path_or_io` is a String, Pathname or File, defaults to basename.
+      #   When `path_or_io` is a IO, defaults to `"stream-{object_id}"`.
+      def initialize(path_or_io, opts = {})
         opts = FormData.ensure_hash(opts)
 
         if opts.key? :mime_type
@@ -42,40 +41,28 @@ def initialize(file_or_io, opts = {})
           opts[:content_type] = opts[:mime_type]
         end
 
-        @file_or_io   = file_or_io
+        @io           = make_io(path_or_io)
         @content_type = opts.fetch(:content_type, DEFAULT_MIME).to_s
-        @filename     = opts.fetch :filename do
-          case file_or_io
-          when String then ::File.basename file_or_io
-          when ::File then ::File.basename file_or_io.path
-          else             "stream-#{file_or_io.object_id}"
-          end
-        end
+        @filename     = opts.fetch(:filename, filename_for(@io))
       end
 
-      # Returns content size.
-      #
-      # @return [Integer]
-      def size
-        with_io(&:size)
-      end
+      private
 
-      # Returns content of a file of IO.
-      #
-      # @return [String]
-      def to_s
-        with_io(&:read)
+      def make_io(path_or_io)
+        if path_or_io.is_a?(String)
+          ::File.open(path_or_io, :binmode => true)
+        elsif defined?(Pathname) && path_or_io.is_a?(Pathname)
+          path_or_io.open(:binmode => true)
+        else
+          path_or_io
+        end
       end
 
-      private
-
-      # @yield [io] Gives IO instance to the block
-      # @return result of yielded block
-      def with_io
-        if @file_or_io.is_a?(::File) || @file_or_io.is_a?(StringIO)
-          yield @file_or_io
+      def filename_for(io)
+        if io.respond_to?(:path)
+          ::File.basename io.path
         else
-          ::File.open(@file_or_io, "rb") { |io| yield io }
+          "stream-#{io.object_id}"
         end
       end
     end
diff --git a/lib/http/form_data/multipart.rb b/lib/http/form_data/multipart.rb
index 27d862d..d51f12d 100644
--- a/lib/http/form_data/multipart.rb
+++ b/lib/http/form_data/multipart.rb
@@ -3,23 +3,21 @@
 require "securerandom"
 
 require "http/form_data/multipart/param"
+require "http/form_data/readable"
+require "http/form_data/composite_io"
 
 module HTTP
   module FormData
     # `multipart/form-data` form data.
     class Multipart
+      include Readable
+
       # @param [#to_h, Hash] data form data key-value Hash
       def initialize(data)
-        @parts          = Param.coerce FormData.ensure_hash data
-        @boundary       = (Array.new(21, "-") << SecureRandom.hex(21)).join("")
-        @content_length = nil
-      end
+        parts = Param.coerce FormData.ensure_hash data
 
-      # Returns content to be used for HTTP request body.
-      #
-      # @return [String]
-      def to_s
-        head + @parts.map(&:to_s).join(glue) + tail
+        @boundary = ("-" * 21) << SecureRandom.hex(21)
+        @io = CompositeIO.new [*parts.flat_map { |part| [glue, part] }, tail]
       end
 
       # Returns MIME type to be used for HTTP request `Content-Type` header.
@@ -33,31 +31,18 @@ def content_type
       # `Content-Length` header.
       #
       # @return [Integer]
-      def content_length
-        unless @content_length
-          @content_length  = head.bytesize + tail.bytesize
-          @content_length += @parts.map(&:size).reduce(:+)
-          @content_length += (glue.bytesize * (@parts.count - 1))
-        end
-
-        @content_length
-      end
+      alias content_length size
 
       private
 
-      # @return [String]
-      def head
-        @head ||= "--#{@boundary}#{CRLF}"
-      end
-
       # @return [String]
       def glue
-        @glue ||= "#{CRLF}--#{@boundary}#{CRLF}"
+        @glue ||= "--#{@boundary}#{CRLF}"
       end
 
       # @return [String]
       def tail
-        @tail ||= "#{CRLF}--#{@boundary}--"
+        @tail ||= "--#{@boundary}--"
       end
     end
   end
diff --git a/lib/http/form_data/multipart/param.rb b/lib/http/form_data/multipart/param.rb
index d49d13f..080b9e2 100644
--- a/lib/http/form_data/multipart/param.rb
+++ b/lib/http/form_data/multipart/param.rb
@@ -1,34 +1,16 @@
 # frozen_string_literal: true
 
+require "http/form_data/readable"
+require "http/form_data/composite_io"
+
 module HTTP
   module FormData
     class Multipart
       # Utility class to represent multi-part chunks
       class Param
-        # @param [#to_s] name
-        # @param [FormData::File, FormData::Part, #to_s] value
-        def initialize(name, value)
-          @name = name.to_s
-
-          @part =
-            if value.is_a?(FormData::Part)
-              value
-            else
-              FormData::Part.new(value)
-            end
-
-          parameters = { :name => @name }
-          parameters[:filename] = @part.filename if @part.filename
-          parameters = parameters.map { |k, v| "#{k}=#{v.inspect}" }.join("; ")
+        include Readable
 
-          @header = "Content-Disposition: form-data; #{parameters}"
-
-          return unless @part.content_type
-
-          @header += "#{CRLF}Content-Type: #{@part.content_type}"
-        end
-
-        # Returns body part with headers and data.
+        # Initializes body part with headers and data.
         #
         # @example With {FormData::File} value
         #
@@ -44,15 +26,19 @@ def initialize(name, value)
         #   ixti
         #
         # @return [String]
-        def to_s
-          "#{@header}#{CRLF * 2}#{@part}"
-        end
+        # @param [#to_s] name
+        # @param [FormData::File, FormData::Part, #to_s] value
+        def initialize(name, value)
+          @name = name.to_s
 
-        # Calculates size of a part (headers + body).
-        #
-        # @return [Integer]
-        def size
-          @header.bytesize + (CRLF.bytesize * 2) + @part.size
+          @part =
+            if value.is_a?(FormData::Part)
+              value
+            else
+              FormData::Part.new(value)
+            end
+
+          @io = CompositeIO.new [header, @part, footer]
         end
 
         # Flattens given `data` Hash into an array of `Param`'s.
@@ -72,6 +58,34 @@ def self.coerce(data)
 
           params
         end
+
+        private
+
+        def header
+          header = String.new
+          header << "Content-Disposition: form-data; #{parameters}#{CRLF}"
+          header << "Content-Type: #{content_type}#{CRLF}" if content_type
+          header << CRLF
+          header
+        end
+
+        def parameters
+          parameters = { :name => @name }
+          parameters[:filename] = filename if filename
+          parameters.map { |k, v| "#{k}=#{v.inspect}" }.join("; ")
+        end
+
+        def content_type
+          @part.content_type
+        end
+
+        def filename
+          @part.filename
+        end
+
+        def footer
+          CRLF.dup
+        end
       end
     end
   end
diff --git a/lib/http/form_data/part.rb b/lib/http/form_data/part.rb
index 9b4a5e9..a1bede8 100644
--- a/lib/http/form_data/part.rb
+++ b/lib/http/form_data/part.rb
@@ -1,5 +1,9 @@
 # frozen_string_literal: true
 
+require "stringio"
+
+require "http/form_data/readable"
+
 module HTTP
   module FormData
     # Represents a body part of multipart/form-data request.
@@ -9,30 +13,18 @@ module FormData
     #  body = "Message"
     #  FormData::Part.new body, :content_type => 'foobar.txt; charset="UTF-8"'
     class Part
+      include Readable
+
       attr_reader :content_type, :filename
 
       # @param [#to_s] body
       # @param [String] content_type Value of Content-Type header
       # @param [String] filename     Value of filename parameter
       def initialize(body, content_type: nil, filename: nil)
-        @body = body.to_s
+        @io = StringIO.new(body.to_s)
         @content_type = content_type
         @filename = filename
       end
-
-      # Returns content size.
-      #
-      # @return [Integer]
-      def size
-        @body.bytesize
-      end
-
-      # Returns content of a file of IO.
-      #
-      # @return [String]
-      def to_s
-        @body
-      end
     end
   end
 end
diff --git a/lib/http/form_data/readable.rb b/lib/http/form_data/readable.rb
new file mode 100644
index 0000000..f85e77d
--- /dev/null
+++ b/lib/http/form_data/readable.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module HTTP
+  module FormData
+    # Common behaviour for objects defined by an IO object.
+    module Readable
+      # Returns IO content.
+      #
+      # @return [String]
+      def to_s
+        rewind
+        read
+      end
+
+      # Reads and returns part of IO content.
+      #
+      # @param [Integer] length Number of bytes to retrieve
+      # @param [String] outbuf String to be replaced with retrieved data
+      #
+      # @return [String, nil]
+      def read(length = nil, outbuf = nil)
+        @io.read(length, outbuf)
+      end
+
+      # Returns IO size.
+      #
+      # @return [Integer]
+      def size
+        @io.size
+      end
+
+      # Rewinds the IO.
+      def rewind
+        @io.rewind
+      end
+    end
+  end
+end
diff --git a/spec/lib/http/form_data/composite_io_spec.rb b/spec/lib/http/form_data/composite_io_spec.rb
new file mode 100644
index 0000000..edc4dd2
--- /dev/null
+++ b/spec/lib/http/form_data/composite_io_spec.rb
@@ -0,0 +1,87 @@
+# frozen_string_literal: true
+
+RSpec.describe HTTP::FormData::CompositeIO do
+  let(:ios) { ["Hello", " ", "", "world", "!"].map { |s| StringIO.new(s) } }
+  subject(:composite_io) { HTTP::FormData::CompositeIO.new(ios) }
+
+  describe "#initialize" do
+    it "accepts IOs and strings" do
+      composite_io = HTTP::FormData::CompositeIO.new ["Hello ", StringIO.new("world!")]
+      expect(composite_io.read).to eq "Hello world!"
+    end
+
+    it "fails if an IO is neither a String nor an IO" do
+      expect { HTTP::FormData::CompositeIO.new [:hello, :world] }
+        .to raise_error(ArgumentError)
+    end
+  end
+
+  describe "#read" do
+    it "reads all data" do
+      expect(composite_io.read).to eq "Hello world!"
+    end
+
+    it "reads partial data" do
+      expect(composite_io.read(3)).to eq "Hel"
+      expect(composite_io.read(2)).to eq "lo"
+      expect(composite_io.read(1)).to eq " "
+      expect(composite_io.read(6)).to eq "world!"
+    end
+
+    it "returns empty string when no data was retrieved" do
+      composite_io.read
+      expect(composite_io.read).to eq ""
+    end
+
+    it "returns nil when no partial data was retrieved" do
+      composite_io.read
+      expect(composite_io.read(3)).to eq nil
+    end
+
+    it "reads partial data with a buffer" do
+      outbuf = String.new
+      expect(composite_io.read(3, outbuf)).to eq "Hel"
+      expect(composite_io.read(2, outbuf)).to eq "lo"
+      expect(composite_io.read(1, outbuf)).to eq " "
+      expect(composite_io.read(6, outbuf)).to eq "world!"
+    end
+
+    it "fills the buffer with retrieved content" do
+      outbuf = String.new
+      composite_io.read(3, outbuf)
+      expect(outbuf).to eq "Hel"
+      composite_io.read(2, outbuf)
+      expect(outbuf).to eq "lo"
+      composite_io.read(1, outbuf)
+      expect(outbuf).to eq " "
+      composite_io.read(6, outbuf)
+      expect(outbuf).to eq "world!"
+    end
+
+    it "returns nil when no partial data was retrieved with a buffer" do
+      outbuf = String.new("content")
+      composite_io.read
+      expect(composite_io.read(3, outbuf)).to eq nil
+      expect(outbuf).to eq ""
+    end
+  end
+
+  describe "#rewind" do
+    it "rewinds all IOs" do
+      composite_io.read
+      composite_io.rewind
+      expect(composite_io.read).to eq "Hello world!"
+    end
+  end
+
+  describe "#size" do
+    it "returns sum of all IO sizes" do
+      expect(composite_io.size).to eq 12
+    end
+
+    it "returns 0 when there are no IOs" do
+      empty_composite_io = HTTP::FormData::CompositeIO.new []
+      expect(empty_composite_io.size).to eq 0
+    end
+  end
+end
diff --git a/spec/lib/http/form_data/file_spec.rb b/spec/lib/http/form_data/file_spec.rb
index dbcda05..608b40d 100644
--- a/spec/lib/http/form_data/file_spec.rb
+++ b/spec/lib/http/form_data/file_spec.rb
@@ -12,9 +12,9 @@
       it { is_expected.to eq fixture("the-http-gem.info").size }
     end
 
-    context "when file given as StringIO" do
-      let(:file) { StringIO.new "привет мир!" }
-      it { is_expected.to eq 20 }
+    context "when file given as a Pathname" do
+      let(:file) { fixture("the-http-gem.info") }
+      it { is_expected.to eq fixture("the-http-gem.info").size }
     end
 
     context "when file given as File" do
@@ -22,6 +22,11 @@
       after { file.close }
       it { is_expected.to eq fixture("the-http-gem.info").size }
     end
+
+    context "when file given as IO" do
+      let(:file) { StringIO.new "привет мир!" }
+      it { is_expected.to eq 20 }
+    end
   end
 
   describe "#to_s" do
@@ -32,16 +37,91 @@
       it { is_expected.to eq fixture("the-http-gem.info").read(:mode => "rb") }
     end
 
-    context "when file given as StringIO" do
+    context "when file given as a Pathname" do
+      let(:file) { fixture("the-http-gem.info") }
+      it { is_expected.to eq fixture("the-http-gem.info").read(:mode => "rb") }
+    end
+
+    context "when file given as File" do
+      let(:file) { fixture("the-http-gem.info").open("rb") }
+      after { file.close }
+      it { is_expected.to eq fixture("the-http-gem.info").read(:mode => "rb") }
+    end
+
+    context "when file given as IO" do
       let(:file) { StringIO.new "привет мир!" }
       it { is_expected.to eq "привет мир!" }
     end
+  end
+
+  describe "#read" do
+    subject { described_class.new(file, opts).read }
+
+    context "when file given as a String" do
+      let(:file) { fixture("the-http-gem.info").to_s }
+      it { is_expected.to eq fixture("the-http-gem.info").read(:mode => "rb") }
+    end
+
+    context "when file given as a Pathname" do
+      let(:file) { fixture("the-http-gem.info") }
+      it { is_expected.to eq fixture("the-http-gem.info").read(:mode => "rb") }
+    end
 
     context "when file given as File" do
       let(:file) { fixture("the-http-gem.info").open("rb") }
       after { file.close }
       it { is_expected.to eq fixture("the-http-gem.info").read(:mode => "rb") }
     end
+
+    context "when file given as IO" do
+      let(:file) { StringIO.new "привет мир!" }
+      it { is_expected.to eq "привет мир!" }
+    end
+  end
+
+  describe "#rewind" do
+    subject { described_class.new(file, opts) }
+
+    context "when file given as a String" do
+      let(:file) { fixture("the-http-gem.info").to_s }
+
+      it "rewinds the underlying IO object" do
+        content = subject.read
+        subject.rewind
+        expect(subject.read).to eq content
+      end
+    end
+
+    context "when file given as a Pathname" do
+      let(:file) { fixture("the-http-gem.info") }
+
+      it "rewinds the underlying IO object" do
+        content = subject.read
+        subject.rewind
+        expect(subject.read).to eq content
+      end
+    end
+
+    context "when file given as File" do
+      let(:file) { fixture("the-http-gem.info").open("rb") }
+      after { file.close }
+
+      it "rewinds the underlying IO object" do
+        content = subject.read
+        subject.rewind
+        expect(subject.read).to eq content
+      end
+    end
+
+    context "when file given as IO" do
+      let(:file) { StringIO.new "привет мир!" }
+
+      it "rewinds the underlying IO object" do
+        content = subject.read
+        subject.rewind
+        expect(subject.read).to eq content
+      end
+    end
   end
 
   describe "#filename" do
@@ -58,10 +138,10 @@
       end
     end
 
-    context "when file given as StringIO" do
-      let(:file) { StringIO.new }
+    context "when file given as a Pathname" do
+      let(:file) { fixture("the-http-gem.info") }
 
-      it { is_expected.to eq "stream-#{file.object_id}" }
+      it { is_expected.to eq ::File.basename file }
 
       context "and filename given with options" do
         let(:opts) { { :filename => "foobar.txt" } }
@@ -80,6 +160,17 @@
         it { is_expected.to eq "foobar.txt" }
       end
     end
+
+    context "when file given as IO" do
+      let(:file) { StringIO.new }
+
+      it { is_expected.to eq "stream-#{file.object_id}" }
+
+      context "and filename given with options" do
+        let(:opts) { { :filename => "foobar.txt" } }
+        it { is_expected.to eq "foobar.txt" }
+      end
+    end
   end
 
   describe "#content_type" do
diff --git a/spec/lib/http/form_data/multipart_spec.rb b/spec/lib/http/form_data/multipart_spec.rb
index bd95614..eb201c4 100644
--- a/spec/lib/http/form_data/multipart_spec.rb
+++ b/spec/lib/http/form_data/multipart_spec.rb
@@ -6,19 +6,6 @@
   let(:boundary)      { /-{21}[a-f0-9]{42}/ }
   subject(:form_data) { HTTP::FormData::Multipart.new params }
 
-  describe "#content_type" do
-    subject { form_data.content_type }
-
-    let(:content_type) { %r{^multipart\/form-data; boundary=#{boundary}$} }
-
-    it { is_expected.to match(content_type) }
-  end
-
-  describe "#content_length" do
-    subject { form_data.content_length }
-    it { is_expected.to eq form_data.to_s.bytesize }
-  end
-
   describe "#to_s" do
     def disposition(params)
       params = params.map { |k, v| "#{k}=#{v.inspect}" }.join("; ")
@@ -36,14 +23,14 @@ def disposition(params)
         "#{crlf}bar#{crlf}",
         "--#{boundary_value}#{crlf}",
         "#{disposition 'name' => 'baz', 'filename' => file.filename}#{crlf}",
-        "Content-Type: #{file.mime_type}#{crlf}",
+        "Content-Type: #{file.content_type}#{crlf}",
         "#{crlf}#{file}#{crlf}",
         "--#{boundary_value}--"
       ].join("")
     end
 
     context "with filename set to nil" do
-      let(:part) { HTTP::FormData::Part.new("s", :filename => nil) }
+      let(:part) { HTTP::FormData::Part.new("s", :content_type => "mime/type") }
       let(:form_data) { HTTP::FormData::Multipart.new(:foo => part) }
 
       it "doesn't include a filename" do
@@ -52,10 +39,60 @@ def disposition(params)
         expect(form_data.to_s).to eq [
           "--#{boundary_value}#{crlf}",
           "#{disposition 'name' => 'foo'}#{crlf}",
+          "Content-Type: #{part.content_type}#{crlf}",
           "#{crlf}s#{crlf}",
           "--#{boundary_value}--"
         ].join("")
       end
     end
+
+    context "with content type set to nil" do
+      let(:part) { HTTP::FormData::Part.new("s") }
+      let(:form_data) { HTTP::FormData::Multipart.new(:foo => part) }
+
+      it "doesn't include a filename" do
+        boundary_value = form_data.content_type[/(#{boundary})$/, 1]
+
+        expect(form_data.to_s).to eq [
+          "--#{boundary_value}#{crlf}",
+          "#{disposition 'name' => 'foo'}#{crlf}",
+          "#{crlf}s#{crlf}",
+          "--#{boundary_value}--"
+        ].join("")
+      end
+    end
+  end
+
+  describe "#size" do
+    it "returns bytesize of multipart data" do
+      expect(form_data.size).to eq form_data.to_s.bytesize
+    end
+  end
+
+  describe "#read" do
+    it "returns multipart data" do
+      expect(form_data.read).to eq form_data.to_s
+    end
+  end
+
+  describe "#rewind" do
+    it "rewinds the multipart data IO" do
+      form_data.read
+      form_data.rewind
+      expect(form_data.read).to eq form_data.to_s
+    end
+  end
+
+  describe "#content_type" do
+    subject { form_data.content_type }
+
+    let(:content_type) { %r{^multipart\/form-data; boundary=#{boundary}$} }
+
+    it { is_expected.to match(content_type) }
+  end
+
+  describe "#content_length" do
+    subject { form_data.content_length }
+    it { is_expected.to eq form_data.to_s.bytesize }
   end
 end
diff --git a/spec/lib/http/form_data/part_spec.rb b/spec/lib/http/form_data/part_spec.rb
index 32a5ce0..8c92b56 100644
--- a/spec/lib/http/form_data/part_spec.rb
+++ b/spec/lib/http/form_data/part_spec.rb
@@ -1,11 +1,12 @@
 # frozen_string_literal: true
 
 RSpec.describe HTTP::FormData::Part do
-  let(:body) { "" }
-  let(:opts) { {} }
+  let(:body)     { "" }
+  let(:opts)     { {} }
+  subject(:part) { HTTP::FormData::Part.new(body, opts) }
 
   describe "#size" do
-    subject { described_class.new(body, opts).size }
+    subject { part.size }
 
     context "when body given as a String" do
       let(:body) { "привет мир!" }
@@ -14,7 +15,7 @@
   end
 
   describe "#to_s" do
-    subject { described_class.new(body, opts).to_s }
+    subject! { part.to_s }
 
     context "when body given as String" do
       let(:body) { "привет мир!" }
@@ -22,8 +23,29 @@
     end
   end
 
+  describe "#read" do
+    subject { part.read }
+
+    context "when body given as String" do
+      let(:body) { "привет мир!" }
+      it { is_expected.to eq "привет мир!" }
+    end
+  end
+
+  describe "#rewind" do
+    context "when body given as String" do
+      let(:body) { "привет мир!" }
+
+      it "rewinds the underlying IO object" do
+        part.read
+        part.rewind
+        expect(part.read).to eq "привет мир!"
+      end
+    end
+  end
+
   describe "#filename" do
-    subject { described_class.new(body, opts).filename }
+    subject { part.filename }
 
     it { is_expected.to eq nil }
 
@@ -34,7 +56,7 @@
   end
 
   describe "#content_type" do
-    subject { described_class.new(body, opts).content_type }
+    subject { part.content_type }
 
     it { is_expected.to eq nil }
 
diff --git a/spec/lib/http/form_data_spec.rb b/spec/lib/http/form_data_spec.rb
index 9798ab8..f75d0e5 100644
--- a/spec/lib/http/form_data_spec.rb
+++ b/spec/lib/http/form_data_spec.rb
@@ -10,14 +10,14 @@
     end
 
     context "when form has at least one file param" do
-      let(:gemspec) { HTTP::FormData::File.new "gemspec" }
-      let(:params)  { { :foo => :bar, :baz => gemspec } }
+      let(:file) { HTTP::FormData::File.new(fixture("the-http-gem.info").to_s) }
+      let(:params) { { :foo => :bar, :baz => file } }
       it { is_expected.to be_a HTTP::FormData::Multipart }
     end
 
     context "when form has file in an array param" do
-      let(:gemspec) { HTTP::FormData::File.new "gemspec" }
-      let(:params)  { { :foo => :bar, :baz => [gemspec] } }
+      let(:file) { HTTP::FormData::File.new(fixture("the-http-gem.info").to_s) }
+      let(:params) { { :foo => :bar, :baz => [file] } }
       it { is_expected.to be_a HTTP::FormData::Multipart }
     end
   end