public static function upload(array &$form, FormStateInterface $form_state) {
    $el = static::el($form, $form_state);

    $form_state->setValueForElement($el['fid'], $el['file']['#value']);
    NestedArray::setValue($form_state->getUserInput(), $el['fid']['#parents'], $el['file']['#value']);

    $form_state->setRebuild();
  }