public function supported_returntypes() {
        // We can only support references if the system account is connected.
        if (!empty($this->issuer) && $this->issuer->is_system_account_connected()) {
            $setting = get_config('googledocs', 'supportedreturntypes');
            if ($setting == 'internal') {
                return FILE_INTERNAL;
            } else if ($setting == 'external') {
                return FILE_CONTROLLED_LINK;
            } else {
                return FILE_CONTROLLED_LINK | FILE_INTERNAL;
            }
        } else {
            return FILE_INTERNAL;
        }
    }