First I must said I love XnView, it's my choice for pic browsing, and the shell extension is the best add-on after picaview from acdsee...
But I miss an option...

I would suggest the option to open a file which is previewed in the context menu by clicking on the thumbnail, with the system associated program instead of XnView.
Very interesting for video files and really fast quick access to one picture,
I would really appreciate these option!
Thanks in advance!