protected function validate_required_file($field, $input, $param = null)
      {
          if (!isset($input[$field])) {
              return;
          }

          if (is_array($input[$field]) && $input[$field]['error'] !== 4) {
              return;
          }

          return array(
              'field' => $field,
              'value' => $input[$field],
              'rule' => __FUNCTION__,
              'param' => $param,
          );
      }