Sending mails with attachments in Magento 2

The Magento 2 Transport Builder, responsible for preparing a mail message, wraps the mail component of the Zend Framework. Unfortunatly it is missing a method to add attachments. I submitted a Pull Request to Magento 2 to fix the issue but for now we need a work-a-round in our project. First we extend the Magento 2 Transport Builder like this:

<?php
namespace bitExpert\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;
    }
}

To make sure Magento 2 will pick our implementation of the TransportBuilder we need to extend the configuration to add a preference setting for the \Magento\Framework\Mail\Template\TransportBuilder type:

<preference for="\Magento\Framework\Mail\Template\TransportBuilder"
    type="\bitExpert\Magento\Mail\Template\TransportBuilder" />