/**
  * Add custom properties for specific post types
  *
  * @param string             $post_type Post type of field that is being added.
  * @param WPSEO_Config_Field $field     Field that corresponds to the post type.
  */
 private function add_custom_properties($post_type, $field)
 {
     if ('attachment' === $post_type) {
         $field->set_property('explanation', __('WordPress automatically generates an URL for each media item in the library. Enabling this will allow for google to index the generated URL.', 'wordpress-seo'));
     }
 }