allgames71, there's an easy way, but it's a little better suited for deploying OS with the custom file type associations set, rather than applying it after the installation.
Basically, you configure the system as you like in your account, then run:
Then you can import XML on any OS
dism /online /Export-DefaultAppAssociations:C:\temp\AppAssoc.xml
But there's a catch! It only applies to accounts created afterwards. The existing accounts don't get these settings, except for the one you used to configure file type associations.
dism /online /Import-DefaultAppAssociations:C:\temp\AppAssoc.xml
To take advantage of this method at deployment time, you mount the image (e.g. install.wim) and import XML into it:
Then you deploy the image and pat yourself on the back
dism /Mount-Image /ImageFile:C:\wim\install.wim /MountDir:C:\mount
dism /Image:C:\mount /Import-DefaultAppAssociations:C:\temp\AppAssoc.xml