The maintenance cleanup task expects the extension without the period. If you put the period in, it will not remove any files.
Also, make sure you have selected the right kind of files to be removed. If you have selected maintenance plan report files, it will not remove backup files.
And finally, if the backups are not native SQL Server backup files - it will not work either. So, if you are using Litespeed, Redgate SQL Backup, or some other third party backup software - those files can only be removed using the third parties cleanup task.