Per-domain banned extensions with pre-queue filter

[Michael Orlitzky]

We have a pre-queue filter set up with a pretty boring config. In postfix,

  smtpd_proxy_filter = localhost:10024

Then in amavis, we do whatever spam and virus scanning we need to do. If
the message is spam/virus, we reject it at SMTP time rather than
bouncing it. Anything else we pass.

We don't block any file extensions by default.

But now, (only) one of our domains wants us to block zip files. Is there
a way to do that for a single domain in a pre-queue filter?