[exim] demime condition vs MIME ACL

In 4.50, I had to specify the WITH_OLD_DEMIME option so my existing checks
for prohibited attached file extensions would continue to work. But the
4.50 spec.txt says the demime condition is deprecated in favor of the MIME
ACL. However, I can't figure out how to check attached file extensions
using a MIME ACL. What am I missing? Thanks.

Don Walker