To get to the question, it is possible to do email attachments with PHP's mail function. Google "PHP email attachment" and you'll find plenty of examples. However, this isn't possible with Drupal's build in mailing function, drupal_mail. I was thinking about doing something like this a while back with the uc_file module, but I decided against due to people being able to forward the email and the size limitations email providers place on attachments. Keep that in mind if these files are supposed to be either exclusive or large.