[exim] system filter filefile type specific "allow / whiteli…

I need a way to allow exceptions to certain attached file extensions.

This works nice for blocking

   # File extension filtering.
   deny message = Sorry we do not accept executables
        condition = ${if match \
                         {${lc:$mime_filename}} \
                         {\N(\.exe)$\N} \
                      {1}{0}}

accept

How can I use a Subject whitelist phrase to allow delivery?

Consider this subject

Subject: Here is the patch let-this-file-pass
----

I would like to test the Subject line for a specific phrase, or better
yet an external list of phrases...

letthisfilepass
Let.this.file.pass
let-this-file-pass

Is this best done with a system filter? I

Thanks,
Gordon