public function extract_files_from_zip($zipfile, $contextid) {
        global $USER;

        $feedbackfilesupdated = 0;
        $feedbackfilesadded = 0;
        $userswithnewfeedback = array();

        // Unzipping a large zip file is memory intensive.
        raise_memory_limit(MEMORY_EXTRA);

        $packer = get_file_packer('application/zip');
        core_php_time_limit::raise(ASSIGNFEEDBACK_FILE_MAXFILEUNZIPTIME);

        return $packer->extract_to_storage($zipfile,
                                    $contextid,
                                    'assignfeedback_file',
                                    ASSIGNFEEDBACK_FILE_IMPORT_FILEAREA,
                                    $USER->id,
                                    'import');

    }