Bouncing email containing attachments

[Jos Chrispijn]

Can you tell me how I can block email from certain users who have 
attachment(s) included?

I want to force senders to use WeTransfer - bouncing their email should 
also create a separate reply asking this user to use WeTransfer instead 
of email (as in attachments).

As attachements in fact are the email itself, should I scan for mime 
headers (or something like that) to trigger this action?

Thanks for your reply.
Jos Chrispijn