ladoga, mentioned elsewhere that instead of always playing with root files; a user file would be a safer option... A defaults.list file with even one line that you require located in /home/user/.local/share/applications/ should override that one line in /usr/share/applications/defaults.list. You can create the file if it doesn't exist and put your own line(s) for those formats you want to open with some other application. Therefore, modifying the root defaults.list isn't required and if anything ever goes wrong you can delete the user file in /home/user/.local/share/applications/