[exim] System_filter.exim

I am hoping that someone knows the answer to this..

I have the system_filter.exim set up currently to reject messages
containing exe,vbs,bat, etc...
However, instead of rejecting them I want to change the file extentions
from .exe to _exe without rejecting them.
Does anyone know hot to do this, and if so what is the syntax to do it.
I would appreciate any help.

Thanks in advanced,
Andrew