how do you determine if a file is a media file?

[Josh Mellicker]

I am working on a handler to determine whether a file is a media  
file. (for OS X and Windows)

First, I check if the file has a proper extension (.mov, .jpg, etc.)  
if so, I assume the extension is correct (a leap of faith perhaps)

Second, I try to get the OS X filetype from the detailed files. Some  
movies properly list "Moov", some don't.

Third, was going to try loading the file into a Player or Image  
object and see if there's an error, but many types of files will load  
into those objects fine (like text files)

So, I'm wondering if anyone has any tips beyond these things...