My list is set to not allow attachments, but Listserv always allows
plain-text attachments.

Now that we're being targeted by spammers, our host software is
stripping out the virus-laden attachment and replacing it with one
called "substitute.txt" and then passing the message through. (Members
are getting infected and sending out infected emails to all their
addresses, or their addresses have been stolen and are being spoofed.)

Needless to say this is upsetting to me and my members, and I've set the
list to plain-text only and no attachments just to avoid dealing with
this type of thing.

I tried adding "substitute.txt" to the Filter template with reject
action, but that didn't work, perhaps because the words aren't in the
message body? Does the Filter have a keyword for attachments? Any other
ideas?

Thanks,
Shap Wolf