Unfortunately, our system has been specifically built not to support attachments. We did this for a number of reasons.
If you have a file that you'd like to give to your customers, we recommend that you host the file on your own server and then place a link to the file within your email. This also means you won't clog your subscribers inbox with attachments they may not even use.
Here's a quick example of how you might promote a PDF document in their email instead of actually attaching it.