On my last session of "exploring" this fantastic application I found this:
While browsing my folders, entering one full of HTML and MHT files, suddely my Firewall alert me that XnView was trying to connect to some internet sites...!!

Than I understood

To prevent this (for enhance security and avoid another problem referred below) I set as "disabled" the thumbnails view for this extensions...
Infact when there's a large number of HTML file type like (with thumbnails-mode "ON" for them), there's also an high possibility of application blocking or crashing... (probably for the high number of connection trials in a same time).

But... considering that XnView is fully able to render the HTML file type thumbnails during the offline sessions, is possible to set (like in the web-browser or email client) an "OFFLINE MODE" to prevent this trials of connection and the crash/block problem related but saving the thumbnails mode "ON" for this problematic files??
I Know: this is a Image Viewer not a Web Browser... but could be a pretty enhancement!!

Bye... and thanks!!!
Dret.