docs: public void asyncUpload(final UpCompletionHandler handler) throws IOException {
          buildParams();
          if (data != null) {
              client.asyncMultipartPost(configuration.upHost(token), params, "file", fileName,
                      data, mime, new StringMap(), new AsyncCallback() {
                          @Override
                          public void complete(Response r) {
                              handler.complete(key, r);
                          }
                      });
              return;
          }
          client.asyncMultipartPost(configuration.upHost(token), params, "file", fileName,
                  file, mime, new StringMap(), new AsyncCallback() {
                      @Override
                      public void complete(Response r) {
                          handler.complete(key, r);
                      }
                  });
      }