public function prepare_package_file() {
        if (empty($this->singlefile) && empty($this->multifiles)) {
            throw new portfolio_caller_exception('invalidpreparepackagefile', 'portfolio', $this->get_return_url());
        }
        if ($this->singlefile) {
            return $this->exporter->copy_existing_file($this->singlefile);
        }
        foreach ($this->multifiles as $file) {
            $this->exporter->copy_existing_file($file);
        }
    }