How are you checking the mime-type? I've never seen a browser that sends a file without that type of header information, but I'm sure they could exist. You can alternatively double-check your image uploads with getimagesize() or a combination of exif_imagetype() and image_type_to_mime_type() (which coincidently also received the award for function name with the most underscores, PHP Grammies 2005)...
-m