Some few examples below provides a better insight to how this is done.
Sample: Loading mail content
<?php
$mailer = new Mailer;
... //other codes here
$mailer->content('mail.body', 'file'); // load as a file
$mailer->sync('client', ['user' => 'foo@mail.com', 'name' => 'foo']);
$mailer->sync('file', ['http://site.com/filepath.ext']); // file attachment