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