$barcodeobj = new TCPDFBarcode($order->order_barcode, 'C128');

$barcode = base64_encode($barcodeobj->getBarcodePNG(1, 30, array(255,255,255)));

Mail::Send(
(int) $order->id_lang, 'order_conf',
Context::getContext()->getTranslator()->trans( 'Order confirmation', array(), 'Emails.Subject', $orderLanguage->locale ),
$data,
$this->context->customer->email,
$this->context->customer->firstname . ' ' . $this->context->customer->lastname,
null,
null,
$barcode,
null,
_PS_MAIL_DIR_,
false,
(int) $order->id_shop
);

$extra_pdf = array();

$extra_pdf['content'] = $barcode.'.png';

$extra_pdf['name'] = 'barcode.png';

$extra_pdf['mime'] = 'image/png';