mime-type related to extension?

Hi guys!

I have a basic form to upload files, my problem is about the mime type.
I use $type= $_FILES[$file_uploaded]['type']; to check the mime-type of the
fila as I want to allow only .jpeg.
I have then a test like:
if  ($type=="image/pjpeg" || $type=="image/jpeg")
{
...
}

Upload working fine if I send "red.jpg".
However if I rename the file in "red", php recognizes it as
application/octet-stream and not jpeg.
I'm working under linux and for me the extension is not required for a file,
I thought the mime-type was encoded in the file itself and was independent
of the extension.
Any clue on that?
Any other php function I should look at?
The cookbook and pear both use $_FILES[ ] ['type'] so I have no idea here.

Thanks,
Melanie

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