Rule to search for specific text in attachment...

Does anyone know if this can be done?

The antiviral that our company uses replaces the infected attachement
with a text file containing the following text:
"This attachment has been replaced with this message because it was
found to contain a virus:"

That's great - but I seem to have nothing to "lock into" then I attempt
to create a rule...

There is nothing unique in the message header, subject, or message body
that I can use either.

The only thing I have to work with is that unique text in the
attachment...

This latest variant of the Sober email worm is killing me - approx 30
messages a day!