If I change filename patterns or description for a file association, or create any new associations, they are forgotten as soon as I close the file associations menu. 

Any changes to application preference are saved. When these changes are saved I get a popup that says "updating system configuration" for a fraction of a second. This doesn't appear when I click apply after changing filename patterns or description.

I made a clean install of Arch with Plasma 5.5 in Virtualbox and was able to reproduce the bug.

Reproducible: Always

Steps to Reproduce:
1. Open system settings, click applications then file associations tab
2. Search for png and select it
3. In filename patterns section, click add, enter '*.newextension' in the textbox
4. Click apply button
5. Click All Settings button in top left corner
6. Click applications then file associations tab
7. Search for png, select it, check filename patterns

Actual Results:  
'*.newextension' is no longer there

Expected Results:  
changes are saved