Fix sttp client content-type header override (#3472)

client/sttp-client/src/main/scala/sttp/tapir/client/sttp/EndpointToSttpClient.scala

@@ -160,6 +160,10 @@ private[sttp] class EndpointToSttpClient[R](clientOptions: SttpClientOptions, ws
       codec: Codec[L, H, CF],
       req: PartialAnyRequest
   ): PartialAnyRequest = {
+    // If true, Content-Type header was explicitly set, so the body's default value
+    // or the codec's media type should not override it.
+    val wasContentTypeAlreadySet = req.header(HeaderNames.ContentType).nonEmpty
+
     val encoded = codec.encode(v)
     val req2 = bodyType match {
       case RawBodyType.StringBody(charset)  => req.body(encoded, charset.name())
@@ -185,7 +189,7 @@ private[sttp] class EndpointToSttpClient[R](clientOptions: SttpClientOptions, ws
         req.multipartBody(parts.toList)
     }
 
-    req2.contentType(codec.format.mediaType)
+    if (wasContentTypeAlreadySet) req2 else req2.contentType(codec.format.mediaType)
   }
 
   private def partToSttpPart[T](p: Part[T], bodyType: RawBodyType[T]): Part[RequestBody[Any]] =

client/sttp-client/src/test/scala/sttp/tapir/client/sttp/SttpClientRequestTests.scala

@@ -37,4 +37,22 @@ class SttpClientRequestTests extends AnyFunSuite with Matchers {
     uri(Some(true)).toString shouldBe "http://localhost?flag"
     uri(Some(false)).toString shouldBe "http://localhost?flag=false"
   }
+
+  test("dynamic content-type header shouldn't be overridden by body's default one") {
+    // given
+    val testEndpoint = endpoint
+      .in("test")
+      .post
+      .in(header[String](HeaderNames.ContentType))
+      .in(byteArrayBody)
+
+    // when
+    val sttpClientRequest = SttpClientInterpreter()
+      .toRequest(testEndpoint, None)
+      .apply(("image/jpeg", Array.empty))
+
+    // then
+    val actual = sttpClientRequest.headers.find(_.is(HeaderNames.ContentType)).get.value
+    actual shouldEqual "image/jpeg"
+  }
 }