False positives "text/plain,.exe"

[Tilman Schmidt]

On our mailserver, Amavis is quarantining a lot of mails claiming that
they contain a banned attachment of type "text/plain,.exe" even though,
when inspecting the quarantined mail, they turn out not to contain any
attachment at all, not even an image or signature, just "text/plain"
and possibly "text/html" within "multipart/alternative".

Most frequent victims of this are mailing list messages from the Python
mailing list, but other quite innocuous individual mails are affected,
too.

This is becoming quite annoying for the users and creates unnecessary
work for the administrators to check and release all those messages.

Is this a known problem? How can it be fixed?

TIA
Tilman