[Exim] filtering by transport?

I am using exim as a mail gateway and have set up two transports - one for
inside -> out and one for outside -> in (the latter sending directly to the
smarthost instead of peeking at MX records).

The customer now wants to filter the messages so that attachments
containing potentially harmful content (*.exe, *.vbs etc) are being
bounced. The system filter available on ftp does exactly that.
Unfortunately, we want to activate the filter only on one transport, ie
leaving messages to the outside world untouched. I could add an additional
check for 'not-to-filter-destination-domains' near the top of said filter -
but is there a more elegant or 'officially recommended' way of achieving
this?

--Ben