code: public FileSender sendFile(final AsyncFile file, final Handler<AsyncResult<Void>> doneHandler) {
      output.group("file", "file", new Handler<OutputGroup>() {
        @Override
        public void handle(OutputGroup group) {
          doSendFile(file, group, doneHandler);
        }
      });
      return this;
    }
docs: Sends a file on the output port.
  
  @param file The file to send.
  @param doneHandler An asynchronous handler to be called once the file has been sent.
  @return The file sender.