I got it!
There are three commands you have to run from PowerShell: (I only tried in admin mode)
Get-AppxPackage Microsoft.Windows.Photos -AllUsers | Reset-AppxPackage
Get-AppxPackage Microsoft.ZuneVideo -AllUsers | Reset-AppxPackage
Get-AppxPackage Microsoft.ZuneMusic -AllUsers | Reset-AppxPackage
Thank you this is really helpful - now you can set default apps for the most common audio and image files, but formats like MP4, OGV, AVI are still not supported (though "Movies & TV" player will open and play them when you click the files open) - in "Default Apps". when you put in ".mp4". ".avi" or ".ogv" in "Set a default for a file type or link type", "+ Choose a default" remains greyed out and you cannot choose a default app to associate the file type.