code: public static WriterCallback from(final InputStream is) {
     return new WriterCallback() {
       @Override
       public void write(OutputStream os) throws IOException {
         ByteStreams.copy(is, os);
       }
     };
    }
docs: Creates a writer callback that copies all the content read from an {@link InputStream} into
  the target stream.
  
  <p>This writer can be used only once.
  @param is the source
  @return the writer callback