from the sendmail faq
in reply to extract data from a form .. and attach a file in sendmail
Subject: Q4.15 -- How do I create attachments with sendmail?
You don't. Sendmail is a mail transfer agent (MTA). Creating e-mail messages, including adding attachments or signatures, is the function of a mail user agent
(MUA). Some popular MUAs include mutt, elm, exmh, Netscape, Eudora and Pine. Some specialized packages (metamail, some Perl modules, etc.) can also be
used to create messages with attachment.
I guess that puts my idea of attaching files to rest .. I was really hoping to avoid using more modules !