How do you add a PDF attachment to an email?
FINALLY! 4 weeks later... 

So the following code ended up working for me:

    $attachment = array(
        'filepath' =&gt; 'sites/default/files/MyFormsOfAssistance.pdf',
        'filename' =&gt; 'MyFormsOfAssistance.pdf',
        'filemime' =&gt; 'application/pdf'
    );


BUT, the key was that in the MailSystem Configuration I had to set the Formatter and Sender to "SMTP Mailer". MimeMail's config automatically pulled it's values from my SMTP's configuration.