Actually MS broke HTTP protocol on that one and it lead to some very interesting exploits. Malicious user could upload a GIF to some site which allowed image uploads. Image would appear normal to a casual observer, except that it was specially crafted so that IE thought it was actually a JavaScript file (because it guessed the file type on its contents and didn't obey server-side set MIME types) and executed it -> XSS.

I don't think Linux does anything remotely like this. It does allow you to use "file" utility to guess the contents of the file, but it doesn't act on it.