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Another method to bypass some server side checks is to edit the shell code and add at the beginning before the < GIF89;a.
Also you could use an intercepting proxy like w3af,Burp proxy... and change the data type... works like a charm in many situations ;)
Try upload the shell as
-shell.php.jpg
-shell.php%00.jpg
-Try using tamper data to edit the post request
-Dont upload c99 first, upload zarabytes uploader script 1st as it is FUD