diff --git a/build.sbt b/build.sbt
index d56b6178d5..cee034d5f4 100644
--- a/build.sbt
+++ b/build.sbt
@@ -146,7 +146,7 @@ val scalaTest = libraryDependencies ++= Seq("freespec", "funsuite", "flatspec",
 val zioVersion = "1.0.3"
 val zioInteropRsVersion = "1.3.0.7-2"
 
-val sttpModelVersion = "1.2.0-RC5"
+val sttpModelVersion = "1.2.0-RC6"
 val sttpSharedVersion = "1.0.0-RC8"
 
 val logback = "ch.qos.logback" % "logback-classic" % "1.2.3"
diff --git a/core/src/test/scala/sttp/client3/testing/HttpTest.scala b/core/src/test/scala/sttp/client3/testing/HttpTest.scala
index f723c8d902..416d18ffa4 100644
--- a/core/src/test/scala/sttp/client3/testing/HttpTest.scala
+++ b/core/src/test/scala/sttp/client3/testing/HttpTest.scala
@@ -383,6 +383,18 @@ trait HttpTest[F[_]]
       req.send(backend).toFuture().map { resp => resp.body should be("p1=v1 (f1), p2=v2 (f2)") }
     }
 
+    "send a multipart message with binary data and filename" in {
+      val binaryPart = {
+        multipart("p1", "v1".getBytes)
+          .fileName("f1")
+      }
+      val req = mp.multipartBody(binaryPart)
+      req.send(backend).toFuture().map { resp =>
+        resp.body should include("f1")
+        resp.body should include("v1")
+      }
+    }
+
     if (supportsCustomMultipartContentType) {
       "send a multipart message with custom content type" in {
         val req = basicRequest
diff --git a/httpclient-backend/src/main/scala/sttp/client3/httpclient/BodyToHttpClient.scala b/httpclient-backend/src/main/scala/sttp/client3/httpclient/BodyToHttpClient.scala
index d50f9fd11f..e32ea346a0 100644
--- a/httpclient-backend/src/main/scala/sttp/client3/httpclient/BodyToHttpClient.scala
+++ b/httpclient-backend/src/main/scala/sttp/client3/httpclient/BodyToHttpClient.scala
@@ -59,21 +59,20 @@ private[httpclient] trait BodyToHttpClient[F[_], S] {
   private def multipartBody[T](parts: Seq[Part[RequestBody[_]]]) = {
     val multipartBuilder = new MultiPartBodyPublisher()
     parts.foreach { p =>
-      val allHeaders = p.headers :+ Header(HeaderNames.ContentDisposition, p.contentDispositionHeaderValue)
+      val allHeaders = Header(HeaderNames.ContentDisposition, p.contentDispositionHeaderValue) +: p.headers
       val partHeaders = allHeaders.map(h => h.name -> h.value).toMap.asJava
-      def fileName = p.fileName.getOrElse("")
-      def contentType = p.contentType.getOrElse(MediaType.ApplicationOctetStream.toString())
       p.body match {
         case NoBody              => // ignore
         case FileBody(f, _)      => multipartBuilder.addPart(p.name, f.toFile.toPath, partHeaders)
         case StringBody(b, _, _) => multipartBuilder.addPart(p.name, b, partHeaders)
         case ByteArrayBody(b, _) =>
-          multipartBuilder.addPart(p.name, supplier(new ByteArrayInputStream(b)), fileName, contentType)
+          multipartBuilder.addPart(p.name, supplier(new ByteArrayInputStream(b)), partHeaders)
         case ByteBufferBody(b, _) =>
           if ((b: Buffer).isReadOnly())
-            multipartBuilder.addPart(p.name, supplier(new ByteBufferBackedInputStream(b)), fileName, contentType)
-          else multipartBuilder.addPart(p.name, supplier(new ByteArrayInputStream(b.array())), fileName, contentType)
-        case InputStreamBody(b, _) => multipartBuilder.addPart(p.name, supplier(b), fileName, contentType)
+            multipartBuilder.addPart(p.name, supplier(new ByteBufferBackedInputStream(b)), partHeaders)
+          else
+            multipartBuilder.addPart(p.name, supplier(new ByteArrayInputStream(b.array())), partHeaders)
+        case InputStreamBody(b, _) => multipartBuilder.addPart(p.name, supplier(b), partHeaders)
         case StreamBody(_)         => throw new IllegalArgumentException("Streaming multipart bodies are not supported")
         case MultipartBody(_)      => throwNestedMultipartNotAllowed
       }
@@ -81,9 +80,9 @@ private[httpclient] trait BodyToHttpClient[F[_], S] {
     multipartBuilder
   }
 
-  private def supplier[T](t: => T) =
-    new Supplier[T] {
-      override def get(): T = t
+  private def supplier(t: => InputStream) =
+    new Supplier[InputStream] {
+      override def get(): InputStream = t
     }
 
   // https://stackoverflow.com/a/6603018/362531
diff --git a/httpclient-backend/src/main/scala/sttp/client3/httpclient/MultiPartBodyPublisher.java b/httpclient-backend/src/main/scala/sttp/client3/httpclient/MultiPartBodyPublisher.java
index 7775150732..47cba27c46 100644
--- a/httpclient-backend/src/main/scala/sttp/client3/httpclient/MultiPartBodyPublisher.java
+++ b/httpclient-backend/src/main/scala/sttp/client3/httpclient/MultiPartBodyPublisher.java
@@ -48,13 +48,12 @@ public MultiPartBodyPublisher addPart(String name, Path value, Map<String, Strin
         return this;
     }
 
-    public MultiPartBodyPublisher addPart(String name, Supplier<InputStream> value, String filename, String contentType) {
+    public MultiPartBodyPublisher addPart(String name, Supplier<InputStream> value, Map<String, String> headers) {
         PartsSpecification newPart = new PartsSpecification();
         newPart.type = PartsSpecification.TYPE.STREAM;
         newPart.name = name;
         newPart.stream = value;
-        newPart.filename = filename;
-        newPart.contentType = contentType;
+        newPart.headers = headers;
         partsSpecificationList.add(newPart);
         return this;
     }
@@ -77,8 +76,6 @@ public enum TYPE {
         String value;
         Path path;
         Supplier<InputStream> stream;
-        String filename;
-        String contentType;
         Map<String, String> headers = new HashMap<>();
     }