Commerce billy mail: how to attach multiple files to an email
I found out, how to attach multiple Attachments:

  $attachments[] = array(
    'filename' =&gt; 'MyDoc.pdf', // can use any alias name here
    'name' =&gt; 'MyDoc.pdf', // required for mandril and other libraries.
    'filemime' =&gt; 'application/pdf', // mime file type
    'type' =&gt; 'application/pdf', // required for mandril and other libraries
    'list' =&gt; TRUE,
    'filepath' =&gt; 'sites/default/files/mail_attachments/MyDoc.pdf',
  );