private function noUpload (BaseModel $model, $fieldName)
  {
    if (!isset ($model[$fieldName])) {
      // Check is there are already files on this field.
      $filesRelation = $model->files ();
      /** @var Builder $filesQuery */
      $filesQuery = $filesRelation->ofField ($fieldName);
      /** @var File[] $files */
      $files = $filesQuery->get ();

      // Delete the previous files of this field, if any exists.
      foreach ($files as $file)
        $file->delete ();
    }
  }