If you have .7z files associated with 7z File Manager, it will open on double click. Is that not the default on installation?
If you choose the wrong one it will not work later.
And it will not repeat the dialog box with the options.
I'll take a look at my setup when I get home tonight.
I usually click "Select all" then unselect ISO. Done.
Edit: other person replied it must be opened in Administrator mode. That explains why I didn't have it associated.