Please make sure that your intro file's size does not exceed the upload_max_filesize, post_max_size, or memory_limit directives in your php.ini file, otherwise PHP will prevent the intro file from ever reaching Centova Cast when you upload it, and will act as if no file was uploaded at all.  This is the most common reason for this problem. 

You can confirm whether this is the problem by returning to the "Files" tab and checking whether an intro/fallback file is set.  If an intro file has been set, you will see the words "Currently: introfile.mp3" directly below the Intro File upload field, with an option to remove the intro file.  If you do not see this, then Centova Cast has not received an intro file.

If you have confirmed that an intro file is indeed set, please check your ShoutCast configuration file in /home/centovacast/vhosts/USERNAME/etc/server.conf and see whether or not the "IntroFile=" line is set, and if so, verify that the intro file actually exists (/home/centovacast/vhosts/USERNAME/var/spool/sounds/introfile.mp3).  If they both exist, then there must be some problem with your ShoutCast server, and/or you have not restarted the server since you uploaded the introduction file.