From the documantation for Mail::Bulkmail:

What about multipart messages? (MIME attachments) *grumble grumble* This is forthcoming, but it won't be in before version 2.5. Maybe 3.0... My current employer absolutely needs a mailing system that can handle attachments, so I figure I might as well finally get around to building it into the module. In the mean time, you can set your own headers, boundaries, etc. and just do the MIME encoding yourself. It will work, I just won't do it for you.

So , bottom line, run your attachment through uuencode first and then e-mail it.