protected function prepareAttachment(array $files) {
    $attachments = NULL;

    foreach ($files as $file) {
      $description = '';

      // If it is an array, this means we also have a description.
      if (is_array($file)) {
        $uuid = key($file);
        $description = current($file);
      }
      else {
        $uuid = $file;
      }

      $object = $this->fileStorage->loadByProperties([
        'uuid' => $uuid,
      ]);

      if ($object) {
        $properties = [
          'target_id' => current($object)->id(),
          'description' => $description,
        ];

        $attachments[] = $properties;
      }
    }

    return $attachments;
  }