protected function migrate_course_files() {
        $ids  = array();
        $fileman = $this->converter->get_file_manager($this->converter->get_contextid(CONTEXT_COURSE), 'course', 'legacy');
        $this->converter->set_stash('course_files_ids', array());
        if (file_exists($this->converter->get_tempdir_path().'/course_files')) {
            $ids = $fileman->migrate_directory('course_files');
            $this->converter->set_stash('course_files_ids', $ids);
        }
        $this->log('course files migrated', backup::LOG_INFO, count($ids));
    }