one of the preffered backdoor .
When ever you see a website for a file upload , just scan it if it has a direct link for file uploading ,if it does than for a example try uploading a image file .
Great if it is uploaded , now try to upload a backdoor .php file.
wolaaa its been uploaded , you can access the whole of it.
Error :Sometimes it may give an error , uploading file, try and tamper the data while uploading the file , make the security low by coping the file and pasteing and editing in notepad .
backdoor available click
if have some dought please ping me on facebook