code: public static Part<byte[]> file(String name, String fileName, byte[] bytes) {
          return new Part<>(name, fileName, bytes, CONTENT_TYPE_BINARY, null, (body, out, charset) -> out.write(body));
      }
docs: Create a file multi-part field, from byte array data.
  This return a part equivalent to &lt;input type="file" /&gt; field in multi part form.