> There is currently a virus attack based on attachments missing file
> extensions..
>
> So instead of virus.zip or virus.exe - the file name is just virus.
Note that MIME filenames are optional on attachments, and for
that matter properly marking MIME parts as having an 'attachment'
content-disposition is optional too. I've seen malware and spam
that cheerfully omits some or all of those.
- cks