RESTesy resource guessing the encoding

resteasy-fileupload-6.java

@POST
@Path("/form")
@Consumes(MediaType.MULTIPART_FORM_DATA)
@Produces(MediaType.TEXT_PLAIN)
public Response uploadForm(MultipartFormDataInput input) throws IOException {
    String charset = input.getFormDataMap().get("_charset_").get(0).getBodyAsString();
    InputPart file = input.getFormDataMap().get("file").get(0);
    InputStream inputStream = file.getBody(InputStream.class, null);
    BufferedReader br = new BufferedReader(new InputStreamReader(inputStream, charset));
    String line;
    StringBuilder content = new StringBuilder();
    while ((line = br.readLine()) != null) {
        content.append(line);
    }
    return Response.ok(content).header("Content-Type", MediaType.TEXT_PLAIN_TYPE.withCharset(charset)).build();
}