Below please find some "loud thoughts" while trying to find a good solution. I haven't found a good solution, yet, perhaps my thoughts are still useful in some way:
Basically we have three actions:
1 - Open in XnView
2 - Open with associated program
3 - No action
And we want to assign file types (.gif, .bmp, .avi, .txt) with these actions.
So the user interface would look like this:
A specific file type can be assigned to only one of the three groups, so one of the groups must not be specified by the user. The group "Open in XnView" is largest (for most users), so "Open in XnView" gets "All other file types":
Now, we still have the problem that specifying the groups is a bit complicated and some people might have to specify many filetypes (e.g. all those people who do not want to open their images in XnView). So this is not a good solution, yet.
Have to think more about this....