public function getChecksum($raw = FALSE) {
        if (isset($this->checksum)) {
            if ($raw) {
                return $this->checksum;
            }
            try {
                return base64_decode($this->checksum);
            } catch (Exception $ex) {
                $this->log('Failed to convert transport document checksum, file might be corrupt: '.
                        $ex->getMessage(), 'Urgent');
            }
        }
        return false;
    }