However, those file types won't display in XnView even though I click them and XnView opens.
.ico, .tiff, etc won't display even after methods mentioned above.

I've tried, but to no avail.
Default programs> Set your default programs > (Program name goes here) > Choose defaults for this program.
Default programs > Associate a file type or protocol with a program
Any help here?
XnView 2.00.0, my XnView settings are stored in the .ini under the XnView installation folder.
Also tried running XnView as Administrator.