It's not possible to change the default application for text files (and perhaps other formats). Changes aren't applied likely because it doesn't request me to type the root password. Changing the application by launching the system settings with root worked.

Reproducible: Always

Steps to Reproduce:
1. Open the File Associations window from the system settings or the "open with..." context menu
2. Try to change the default application for simple text files to another program
3. Apply changes and double-click a text file

Actual Results:  
The file type is still associated with the previous program. If you open the settings again, your customized entry is gone.


I believe that it was supposed to ask for root privileges to apply the changes correctly, because setting a default application by opening the system settings as root worked.