Configuring supported extensions

For example, say .rar isn't supported natively. I've seen tutorials suggest
installing related packages, but don't see how Amavis picks up on that.
From the amavsid.conf, I see a "@decoders" which seems like a map
(extension, command, arguments). This appears to map to
/lib/Amavis/Unpackers.pm.

   - What's the process for ensuring certain extensions get scanned?
   - How do I know which extensions are supported?