resteasy-multipart with @MultipartForm does not consult ParamConverterProvider

While implementing REST-Services in a quarkus application we came across
the problem that when doing fileuploads who are require the use of
resteasy-multipart-provider we can not use custom converters registered
for our POJOs (well in our REST-APIs we only have interfaces).

So what's the problem if we have an REST-Endpoint like this:

@POST
@Produces(MediaType.TEXT_PLAIN)
@Consumes(MediaType.MULTIPART_FORM_DATA)
@Path("multi-interface")
public String multiPartInterface(@MultipartForm MPDataInterface data) {
  return data.metaData.getMetaInfo();
}

public static class MPDataInterface {
  @FormParam("document")
  @PartType(MediaType.APPLICATION_OCTET_STREAM)
  public InputStream document;        

  @FormParam("metaData")
  public IMetaData metaData;
}

where IMetaData looks like this:

package at.bestsolution.quarkus;public interface IMetaData {
  String getMetaInfo();
}

The upload fails because there's not way for REST-Easy to convert from the JSON-Encoded metaData to Java-Object (that's expected).

Because of that we register "ParamConverterProvider" [1] for those
types. This works great for @PathParam(), @QueryParam but fails for the
@MultipartForm.