First of all, you can't trust $_FILES['file_up']['type'] to be accurate: it only says what the uploader claims to be the file type. [man]getimagesize[/man] is a more robust way of determining image file format.
Secondly, the MIME type for a JPEG file is "image/jpeg". Except when it's "image/pjpeg". Although I have seen some clients (incorrectly) use "image/jpg". And once a JPEG under a MIME type of "image/jfif" just to keep things interesting. Not to mention lazy/braindead clients that just don't bother and stick everything under "application/octet-stream" or even "text/plain". Like I said, there are more robust ways of determining an image's format.
