public function getUploadFieldName($field = null)
    {
        if (!is_null($field)) {
            return $field;
        }

        $imagesFields = $this->getDefinedUploadFields();
        $fieldKey = array_first(array_keys($imagesFields));

        // return first field name
        return is_int($fieldKey)
            ? $imagesFields[$fieldKey]
            : $fieldKey;
    }