Can't configure Content-Type for Multipart Form Uploading

Play Version (2.5.x / etc)

2.5.10

API (Scala / Java / Neither / Both)

Java

Operating System (Ubuntu 15.10 / MacOS 10.10 / Windows 10)

Ubuntu 16.04.1 LTS

JDK (Oracle 1.8.0_72, OpenJDK 1.8.x, Azul Zing)

1.8.0_121

Expected Behavior

Following the spec set by RFC 2388 a POST with Content-Type=multipart/form-data should be able to support multiple content-types and character encoding in all of its parts.

Actual Behavior

https://www.playframework.com/documentation/2.5.x/JavaWS#Submitting-multipart/form-data

The play.mvc.Http.MultipartFormData.DataPart doesn't allow any sort of content-type configuration and it defaults to text/plain with no charset, which our back-end (JAX-RS) interprets as the default for text/plain: US-ASCII. RFC 1341
The play.mvc.MultipartFormatter.boundaryToContentType() beeing called uppon the format of a multipart Post also forces a content-type in the header without a charset configuration and therefore when using play.libs.ws.WSRequest.setContentType("multipart/form-data; charset=UTF-8") will make the request carry 2 Content-type entity headers.

#6928 seems like it had a similar problem.

Example Request sent by

POST /multipartform HTTP/1.1
Cache-Control: no-cache
Accept-Language: en
Content-Type: multipart/form-data; boundary=LK9jYpDyGz-nbNxWCA
Accept-Encoding: gzip,deflate
Transfer-Encoding: chunked
Host: 10.112.98.101:8010
Accept: /
User-Agent: AHC/2.0

161
--LK9jYpDyGz-nbNxWCA
Content-Disposition: form-data; name="request"
Content-Type: text/plain

{"stuff":null,"dummyDate":1487772284795,"action":null,"stuff":null}
6c

--LK9jYpDyGz-nbNxWCA
Content-Disposition: form-data; name="filename"
Content-Type: text/plain

SUM.csv
8d

--LK9jYpDyGz-nbNxWCA
Content-Disposition: form-data; name="data"; filename="SUM.csv"
Content-Type: multipart/form-data; charset=UTF-8

13
1;1;1;1;1;1;1;1;1;1
18

--LK9jYpDyGz-nbNxWCA--
0

Start with play.mvc.Http.MultipartFormData.DataPart and play.mvc.MultipartFormatter.boundaryToContentType(), then trace how WSRequest.setContentType() is applied to multipart requests. Done means multipart parts can specify their content types and the request does not emit duplicate Content-Type headers.