Updates :: Last Update Caused All Custom File Associations To Reset To Defaults

My PC updated Windows 10 yesterday afternoon ("Cumulative Update for Windows 10 Version 1511 for x64-based Systems (KB3135173)". After rebooting, it completely screwed all my custom file associations (mostly for media files: photos, audio, and video).

A few minutes after the reboot, the Action Center started firing popups at me, one after the other, every few seconds:

"An app default was reset. An app caused a problem with the default setting for .avi files so it was reset to Films & TV."

with a different file extension for each popup (avi, bmp, mp4, and so on) . Even .pdf files were changed from Adobe Reader to Microsoft Edge. As well, all my custom context menu items for these same file types were also deleted. I spent a lot of time setting these associations up and now they were all obliterated. I'd backed them all up in a .reg file but running it didn't do anything. So with a heavy (and angry) sigh I started manually changing the associations back to the way I had them. However, each time I finished making a change within seconds the Action Center would fire the same message as before at me for the file type I'd just changed and it would be reset back to the MS default again.

Finally in exasperation, I just used System Restore to go back before the updates and that solved the problem. Now, the Action Center keeps nagging me to set a time to reinstall this same update. I know I can't block it indefinitely, and there are probably important security patches in it I should have. So the question is: How can I stop it from f'ing up my file associations again?