Swiftmailer create attachment

gistfile1.php

<?php
//Using composer to get the Sabre lib
require dirname ( __FILE__ ) . '/../vendor/autoload.php';
use Sabre\VObject\Component\VCalendar;

    // Create a message
    $message = Swift_Message::newInstance ( 'Message Title' )
        ->setFrom ( array( '[email protected] ' => 'John Doe' ) )
        ->setTo ( array( $email ) )
        ->setSubject ( "Cool message subject" )
        ->setBody ( $body )
        ->setContentType ( 'text/html' );

    //Set the 'from' to something a bit nicer
    $from = $message->getHeaders ()->get ( 'From' );

    $from->setNameAddresses ( array(
        '[email protected] ' => 'John Doe',
    ) );

    $headers = $message->getHeaders ();

    //Add text header for Mailgun campaign tracking
    $headers->addTextHeader ( 'X-Mailgun-Campaign-Id', '[mailgun-campaign-id]]' );


    //Create calendar invite with Sabre VOject lib
    $cal = new VCalendar();

    //Create a meeting invite that lasts 2 hours on the same day
    $vevent = $cal->add ( 'VEVENT', array(
        'summary' => $subject,
        'location' => 'Meeting room 1',
        'dtstart' => new DateTime( $mtg_date . ' 09:00:00', new \DateTimeZone( 'America/Toronto' ) ),
        'dtend' => new DateTime( $mtg_date . ' 11:00:00', new \DateTimeZone( 'America/Toronto' ) ),
        'method' => 'REQUEST'
    ) );

    //Make ical
    $data = $cal->serialize ();

    $filename =  "invite.ics";

    //Attach the calendar invite to the mail
    $attachment = Swift_Attachment::newInstance ()
        ->setFilename ( $filename )
        ->setContentType ( 'text/calendar' )
        ->setBody ( $data );

    $message->attach ( $attachment );

    $result = $mailer->send ( $message );