I am trying to link a file type to an app and it's not working. The file type does not show up in the list of known file types and when I go to "Open With" it does not save the app. This issue is not system wide and only happens with an EU's profile.
All information on associated file types is stored in the registry under the HKEY_CLASSES_ROOT Key, as described here.
So your file type has to be there, or the association will not work.