public static function clean_file_extensions($extensions) {

        debugging('The method workshop::clean_file_extensions() is deprecated.
            Please use the methods provided by the \core_form\filetypes_util class.', DEBUG_DEVELOPER);

        $extensions = self::normalize_file_extensions($extensions);

        foreach ($extensions as $i => $extension) {
            $extensions[$i] = ltrim($extension, '.');
        }

        return implode(', ', $extensions);
    }