Hello;
go to your ACP >> Settings >>Upload settings >> Valid file extensions and check how many medias are allowed.
I guess you have only "php".
The extensions should be by default:
jpg,gif,png,aif,au,avi,mid,mov,mp3,mpg,swf,wav,ra,rm,zip,pdf
Check also for an additional "alien" php file in your website, as "r57shell.php" and delete it (possible bot hijack).