One of the tools in ProDiscover does that, but I can't think of anything free (which is what I'm assuming you are looking for)

But now that I think about it, how hard would it be to write a little app that would do it? The data sig is in the first 20 bits, and compare it to the file extension. Sounds like that may actually be a fun little project.

Link for ProDiscover: http://www.techpathways.com/DesktopD...dex=3&tabid=12