[exim] Block unwanted extension

hello,
I try to block unwanted extension with the following:

deny
message = Unwanted file extension ($found_extension). Error
$demime_errorlevel
!demime = pdf:txt
log_message = Mail contain unwanted file extension ($found_extension).

insead of list all the unwanted extentions i think it would be better to
list all the "wanted" extention, the problem in the config above is that
messages with no attachment (plain text) is also rejected.

Regards