[exim] Exim rule for only accepting messages with picture at…

Hi,

I'm trying to have a rule for exim to do the following for all
messages send to a particular address (say photo@???):
- check if the email has a picture attachment
- reject if it doesn't and send an email to the sender explaining
the rejection
- send the message to a command using the pipe transport

I read the documentation a bit, and don't really know where to begin
to do that so if anybody has any pointers I would be very grateful.

Thanks,
Tomas