Allow Specific File Extension

[JaymeH]

OK. I'm a tad frustrated. I've been searching for an hour now on both the Zimbra site and google and I have yet to find a satisfying answer. Perhaps I'm asking the wrong question or my search skills suck.

Is there a way to allow a specific file extension through the system without amavis going all heart attack-ish? For instance I would like to "white-list" blahblah.smo globally but I still want to block all the .exe and other baddies. Any help would be much appreciated.