urgend file type detection

Hi

I want to allow only mail through my sarge exim 4.34 heavy daemon server
with clamav that have
- NO attachments
- ONLY rtf, doc, pdf, txt, xls attachments Everything else should be
blocked.

However, I do not want to determine this by extension, but by content.
rfc, doc, pdf als xls should be detectable, as they have special file
structures. txt will propably more complicated.

Has anyone done this before? Where can I start? What can I do?

Regards,
  Steffen