Make your method return Response and set the media type of the response yourself:
@POST
@Consumes(MediaType.MULTIPART_FORM_DATA)
public Response post(
@FormDataParam("part") String s,
@FormDataParam("part") FormDataContentDisposition d) {
final Map<String, String> parameters = Maps.newHashMap();
parameters.put("boundary", "myboundary");
final MediaType mediaType = new MediaType("multipart", "form-data", parameters);
return Response
.ok(s + ":" + d.getFileName(), mediaType)
.build();
}