Magento – How to send transactional mail with csv/excel attachment in magento 2

It appears that Magento 2 Email's TransportBuilder class doesn't support email attachments yet.

Since the default Message class inherits from Zend_Mail class, one workaround is to extend the TransportBuilder class and call $message's createAttachment method to send file attachment in email.

You can use following class in place of TransportBuilder class to build Transport object and include attachment.

use Magento\Framework\Mail\MessageInterface;
use Magento\Framework\Mail\TransportInterfaceFactory;
use Magento\Framework\ObjectManagerInterface;
use Magento\Framework\Mail\Template\FactoryInterface;
use Magento\Framework\Mail\Template\SenderResolverInterface;
use Magento\Framework\Mail\Template\TransportBuilder;


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 $this;
    }

}

M2 does not come with out of the box however it is a feature built into the zend framework. Here is a good reference how to add this functionality into magento: https://blog.bitexpert.de/blog/sending-mails-with-attachments-in-magento-2/

In case link goes dead, create the following

<?php
namespace Your\CustomModule\Magento\Mail\Template;

class TransportBuilder 
    extends \Magento\Framework\Mail\Template\TransportBuilder
{
    public function addAttachment(
        $body,
        $mimeType    = Zend_Mime::TYPE_OCTETSTREAM,
        $disposition = Zend_Mime::DISPOSITION_ATTACHMENT,
        $encoding    = Zend_Mime::ENCODING_BASE64,
        $filename    = null
    ) {
        $this->message->createAttachment($body, $mimeType, $disposition, $encoding, $filename);
        return $this;
    }
}

then add to etc/di.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="\Magento\Framework\Mail\Template\TransportBuilder"
                type="\Your\CustomModule\Magento\Mail\Template\TransportBuilder" />
</config>

Now you can use addAttachment() throughout your site.