I tried to upload an attachment via Multipart. For that i use the client proxy api.

The proxy interface has a method like this:

@Path("test/{key}")
@Consumes("multipart/form-data")
@POST
public void postAttachment(@MultipartForm Attachment attachment, @PathParam("key") String key)

When i generate the proxy with the old ProxyFactory it can send the POST, but an exception is thrown with the new ResteasyClientBuilder:

javax.ws.rs.ProcessingException: could not find writer for content-type multipart/form-data type: de.markant.anwendkart.client.Attachment

The problem seems to be, that the variable "annotations" in ResteasyProviderFactory.getClientMessageBodyWriter() is not correctly filled with a proxy generated by the new ResteasyClientBuilder. Without it, the correct MessageBodyWriter(MultipartFormAnnotationWriter) is not used.

The test project i used is attached.