private static function get_files($assignment, $userid, $attemptnumber, $filearea, $filepath = '/') {
        $grade = $assignment->get_user_grade($userid, true, $attemptnumber);
        $itemid = $grade->id;
        $contextid = $assignment->get_context()->id;
        $component = self::COMPONENT;
        $fs = get_file_storage();
        $files = $fs->get_directory_files($contextid, $component, $filearea, $itemid, $filepath);
        return $files;
    }