code: public static Part<File> file(String name, String fileName, File file) {
          return new Part<>(name, fileName, file, ContentTypes.probeContentType(file), null, (body, out, charset) -> {
              try (InputStream in = new FileInputStream(body)) {
                  InputStreams.transferTo(in, out);
              }
          });
      }
docs: Create a file multi-part field, from file.
  This return a part equivalent to &lt;input type="file" /&gt; field in multi part form.