Il mime type che ti ritrovi nelle variabili $_FILES viene inviato dal client, ergo vale quando i soldi del monopoli.

One should *NOT* upload untrusted files into your web tree, on any server.

Nor should any directory within your web tree have permissions sufficient for an upload to succeed, on a shared server. Any other user on that shared server could write a PHP script to dump anything they want in there!

The $_FILES['userfile']['type'] is essentially USELESS.
A. Browsers aren't consistent in their mime-types, so you'll never catch all the possible combinations of types for any given file format.
B. It can be forged, so it's crappy security anyway.

One's code should INSPECT the actual file to see if it looks kosher.

For example, images can quickly and easily be run through imagegetsize and you at least know the first N bytes LOOK like an image. That doesn't guarantee it's a valid image, but it makes it much less likely to be a workable security breaching file.

For Un*x based servers, one could use exec and 'file' command to see if the Operating System thinks the internal contents seem consistent with the data type you expect.