diff --git a/lib/rack/test.rb b/lib/rack/test.rb
index aba7b3d..412fff9 100644
--- a/lib/rack/test.rb
+++ b/lib/rack/test.rb
@@ -299,9 +299,10 @@ def env_for(uri, env)
         elsif !env.key?(:input)
           env['CONTENT_TYPE'] ||= 'application/x-www-form-urlencoded'
           params ||= {}
+          multipart = env.has_key?(:multipart) ? env.delete(:multipart) : env['CONTENT_TYPE'].start_with?('multipart/')
 
           if params.is_a?(Hash)
-            if data = build_multipart(params)
+            if data = build_multipart(params, false, multipart)
               env[:input] = data
               env['CONTENT_LENGTH'] ||= data.length.to_s
               env['CONTENT_TYPE'] = "#{multipart_content_type(env)}; boundary=#{MULTIPART_BOUNDARY}"
diff --git a/spec/rack/test_spec.rb b/spec/rack/test_spec.rb
index 5ceae15..7ab04d8 100644
--- a/spec/rack/test_spec.rb
+++ b/spec/rack/test_spec.rb
@@ -102,6 +102,27 @@
     last_request.POST['foo'].must_equal 'bar'
   end
 
+  it 'does not use multipart input for :params for POST by default' do
+    request '/', method: :post, params: { 'foo' => 'bar' }
+    last_request.POST['foo'].must_equal 'bar'
+    last_request.env['rack.input'].rewind
+    last_request.env['rack.input'].read.must_equal 'foo=bar'
+  end
+
+  it 'supports :multipart when using :params for POST to force multipart input' do
+    request '/', method: :post, params: { 'foo' => 'bar' }, multipart: true
+    last_request.POST['foo'].must_equal 'bar'
+    last_request.env['rack.input'].rewind
+    last_request.env['rack.input'].read.must_include 'content-disposition: form-data; name="foo"'
+  end
+
+  it 'supports multipart CONTENT_TYPE when using :params for POST to force multipart input' do
+    request '/', method: :post, params: { 'foo' => 'bar' }, 'CONTENT_TYPE'=>'multipart/form-data'
+    last_request.POST['foo'].must_equal 'bar'
+    last_request.env['rack.input'].rewind
+    last_request.env['rack.input'].read.must_include 'content-disposition: form-data; name="foo"'
+  end
+
   it 'supports sending :query_params for POST' do
     request '/', method: :post, query_params: { 'foo' => 'bar' }
     last_request.GET['foo'].must_equal 'bar'