Not *exactly* what you want but quite as effective:
I use RenAttach (
http://freshmeat.net/projects/renattach/) as a *transport*
filter. It will, based on a 'bad list', rename all attachments with a given
extension to something harmless|obvious.
In my configuration, all potentially executable attachments will be renamed
from foo.vbs into foo_vbs.
The user will have to 'save as' into the appropriate form and execute only
then. If the user has some antivirus installed, it should be checked upon
saving.
--Ben
