Ah, the second suffix to a filename trick again. Had this last summer with something similar and went through extensive testing with a mate. If you use a later version of Outlook (2000 onwards and not the express varient) it should block the attachment with the default security rules, but it DID NOT. Outlook Express did mark the attachment as unavaible if the "do not allow an attachment to be opened that may be a virus" box was ticked in the options. The problem was that it blocked almost every damned file.
Keep your AV scanners upto date!
Keep your AV scanners upto date!