public function getMailMergeFieldNames($fileName)
    {
        //check whether file is set or not
        if ($fileName == '')
            throw new Exception('No file name specified');

        $strURI = Product::$baseProductUri . '/words/' . $fileName . '/mailMergeFieldNames';

        $signedURI = Utils::sign($strURI);

        $responseStream = Utils::processCommand($signedURI, 'GET', '', '');

        $json = json_decode($responseStream);

        return $json->FieldNames->Names;
    }