[Exim] exim system filters

We are looking for a solution to allow us the ability to filter a
message based on the content type header in exim4. If it matches a
certain regex, we want to pass the entire message off to a standalone
program we have written. This program would then modify the message and
resubmit it back to exim for delivery. Exim cannot wait for our program
to finish because it could take a few minutes for our program to
complete. We have looked at the pipe transport but it seems to not call
our program. We have placed in exim4.conf.template lines for
system_filter = /etc/exim4/exim4.filter and system_filter_pipe_transport
= our_program. In the our_program: declaration we have entries for
driver = pipe and command = /usr/bin/our-program. Is this the best way
to accomplish our goal? Would a different transport method work better?
The message would need to be passed to our program right before local
delivery to users.

-Chris