Hmm, that would be a fun little project. On *nix there is a command-line tool called file which identifies file-types. So it would be a matter of a good script that sends the right arguments to file and comparing that against a list of extenstions.

As with all utilities check for any know exploits on the version you are using and update to the safe version.

Whoohoo, score one for SourceForge.

The unix file utility for Windows:
http://gnuwin32.sourceforge.net/packages/file.htm

You might find other fun utilities from the rest of that project:
http://gnuwin32.sourceforge.net/

So really, it would still be a matter of writting some kind of script that compares the results from file to the extension of the file in question and returns true or false appropriately.

Ofcourse, as it's a sourceforge progect, you could just edit the code and make your own utility.

Hope that helps,
Dhej