This is simple tip about how to change default applications (web browser, mail client, video/music player, image viewer) in Ubuntu 11.10 Oneiric.
There is no more “Preferred Applications” in Ubuntu Oneiric. These settings are built into System Info managed by System Settings(Gnome Control Center).
In Unity or Gnome-shell, click on your User Account at top-right corner and choose System Settings to launch gnome-control-center.
Then scroll down and click on System Info, navigate to Default Applications tab and then you know what to do.