Simplified Spam Reject filter

[mjpierce]

I download headers every day. It's the only way I know how to figure out what "abcabcabc.7z or .rar" posts really are.
I think that what I want is to, as a "Header Filter" (in this order):
1) Reject bad posters.
2) Accept posts with subjects containing filename extensions that I know and want.
3) Reject everything else.

I know how to do each thing, but not together and in that order.
Is this possible?
Is this a bad idea?

Thanks no matter what.