How do you programatically find out what word's default save format is?

Normally you can assume that word 2003 and before are .doc and 2007 and after are .docx, but on 2007 onwards the user can change this back to .doc

When I call:
Code:
wdoc.saveas("thisname")
It saves properly for everyone, as either thisname.doc or thisname.docx

However if there is a fullstop
Code:
wdoc.saveas("this.name")
I get a file called this.name which does not automatically open with word, I have to choose word for it.

I would like my solution to be to append the correct extension to the filename
Code:
myext$ = iif(<<test goes here>>, ".doc", ".docx")
wdoc.saveas("this.name" & myext$)
Actually, I'd like never to have full stops in filenames, but its a requirement I've been given. Any Ideas?