RWMB_Media_Field::normalize PHP Method

    public static function normalize($field)
    {
        $field = parent::normalize($field);
        $field = wp_parse_args($field, array('std' => array(), 'mime_type' => '', 'max_file_uploads' => 0, 'force_delete' => false, 'max_status' => true));
        $field['multiple'] = true;
        return $field;
    }

 /**
  * Normalize parameters for field
  *
  * @param array $field
  *
  * @return array
  */
 static function normalize($field)
 {
     $field = parent::normalize($field);
     $field['mime_type'] = 'image';
     return $field;
 }