Many many years ago I saw someone using an image hoster which only checked mime type, and not filename. That's the important bit after all right? Uploading an image as image.php worked, and if the exif comment contained php code, it ran.
For server implementations that aren't braindead, that is indeed the important bit. Computers don't inherently know how to run PHP. If the request handler doesn't look at the file extension to decide whether or not to pass the contents to the PHP interpreter (if PHP is even installed on the system), then image.php isn't going to run any PHP.