use MagentoFrameworkObjectManagerInterface;
use MagentoFrameworkMailTemplateFactoryInterface;
use MagentoFrameworkMailTemplateSenderResolverInterface;
use MagentoFrameworkMailTemplateTransportBuilder;
class UploadTransportBuilder extends TransportBuilder {
    public function __construct(FactoryInterface $templateFactory,
        MessageInterface $message,
        SenderResolverInterface $senderResolver,
        ObjectManagerInterface $objectManager,
        TransportInterfaceFactory $mailTransportFactory) {
        parent::__construct($templateFactory,
            $message,
            $senderResolver,
            $objectManager,
            $mailTransportFactory);    }
    public function attachFile($file, $name) {
        if (!empty($file) && file_exists($file)) {
            $this->message
            ->createAttachment(
                file_get_contents($file),
                Zend_Mime::TYPE_OCTETSTREAM,
                Zend_Mime::DISPOSITION_ATTACHMENT,
                Zend_Mime::ENCODING_BASE64,
                basename($name));
            return true;  }
 return false;
    }
}

$transportBuilder = $this->_objectManager->create('MagentoFrameworkMailTemplateUploadTransportBuilder');
        $path='D:asd.csv';
        //end here
        $templateId=1;
        $storeId=1;
        $templateParams['test']='test test';
        $transport =$transportBuilder->setTemplateIdentifier($templateId)
            ->setTemplateOptions(['area' => 'frontend', 'store' => $storeId])
            ->setTemplateVars($templateParams)
            ->setFrom(array('email'=>'anand@gmail.com', 'name'=>'anand'))
            ->addTo('anand.ontigeri@alshaya.com','anand.ontigeri')
            ->getTransport();
        $transport->attachFile($path,'asd');
        $transport->sendMessage();

$transport =$transportBuilder->setTemplateIdentifier($templateId)
            ->setTemplateOptions(['area' => 'frontend', 'store' => $storeId])
            ->setTemplateVars($templateParams)
            ->setFrom(array('email'=>'anand@gmail.com', 'name'=>'anand'))
            ->addTo('anand.ontigeri@alshaya.com','anand.ontigeri')
            ->getTransport();
        $transport->attachFile($path,'asd');
        $transport->sendMessage();

$transport =$transportBuilder->setTemplateIdentifier($templateId)
            ->setTemplateOptions(['area' => 'frontend', 'store' => $storeId])
            ->setTemplateVars($templateParams)
            ->setFrom(array('email'=>'anand@gmail.com', 'name'=>'anand'))
            ->addTo('anand.ontigeri@alshaya.com','anand.ontigeri')
            ->attachFile($path,'asd') //Add attachFile here
            ->getTransport();
        $transport->sendMessage();