Hmm I always learned to also check the mimetype because people would be able to just upload any file thats a .jpg for example.

Of course I don't know how that could harm you server in any way since it won't be read as (for example) an executable. But it doesn't harm adding in your script so I've done it ever since.

I never had problems with finding the mimetypes from a file though. In IE6/7 FF1/2 and Opera 9 I could just upload the files and the mimetype would be found.

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