Just a quick post... In my SharePoint deployment at a client, I included a little maintenance plan on the SQL sever.
After a few weeks I got a call from the client: DUDE ! Our SQL Server disk is FULL !
Checking the usual suspect (logfiles not cleared during backup) I quickly realized that the problem was the 'cleanup task' in the maintenance plan wasn't deleting the old SQL backup files.
For file extension, I used the value ".bak", you shouldn't add the . in there.. just type "bak".
Problemo solved !