Thanks for the suggestion DonQuicky, but yeah was looking to attach a file from the server.
I've managed to get it working after reinstalling a new fresh version of modx on a linux server and playing with the path (the control panel path was different than what the documentation said), but the localhost server path was correct but still doesn't work for some reason.
Anyway, it's working on the live site now; thanks for the help guys!
For anyone else looking, this is the attachment snippet on Plesk 17.0.17 (Onyx):
<?php
$attachment = '/var/www/vhosts/website.com/sub.website.com/assets/uploads/picture.jpg';
$hook->modx->getService('mail', 'mail.modPHPMailer');
$hook->modx->mail->mailer->AddAttachment($attachment);
return true;