I remember finding this recently, hope it helps.

From Mail::Sender pod:

Please keep in mind though that it's not currently possible to create nested parts with Mail::Sender. If you need that level of control you should try MIME::Lite.

But, if you click on the Request Tracker icon, you will note in the correspondence that a function to compose multipart messages with attachments will be added in the future and for now is available in the beta version available (click on the attachment download button).

Hope this helps.