Actually, MS only guess the content-type if it is not sent by the webserver, or if it is one of 26 "known" types.

http://msdn2.microsoft.com/en-us/library/ms775147.aspx

Why? Well, that's more infinite wisdom from Microsoft, in order to "make it easier for an average Joe to put up a personal website without worrying about mimetype details"

http://blogs.msdn.com/ie/archive/2005/02/01/364581.aspx

It's a shame that Gupta doesn't recognise that most websites are put up by professionals*, and that their perhaps well intentioned code is a frigging nightmare at times. "Asking everybody to fix their servers" is precisely what they should do. We expect Microsoft to fix their software, adhere to standards, &c, and they have a right, nay duty, to expect the same in return.

* Insert some reference to professionals using apache and amateurs using IIS here

("infinite wisdom" is a registered trademark of Microsoft Corporation ... well, probably)