I'd like very much being able to exclude (or explicitly include) certain files types from being loaded in the database, not only certain folders. I know there's the user defined filter for the file list, but that is something slightly different. I'm thinking for ex about the jpg_original files created by other programs like Exif GUI etc. I don't mind seeing them in the browser, it's even useful to clean them up there after checking the modified image files, but they are really of no use in the database. Same goes for the swp files created sometimes by Lightroom.
BTW, I even saw files like thumbs.db in the database
Allowing only the really needed file types in the DB would keep it from getting cluttered and bloated with useless data. The DB can be optimized, but it's a bit cumbersome to run an optimization every time I do something with a number of images in Exif GUI for ex. (And yes, I am a DB applications developer 
.)And BTW, a big thank you to Pierre, XnView is a really great software, and MP is a big leap forward. I've chosen it some years ago, because it was the only DAM recognising and showing at least some of the proprietory formats of Paintshop Pro.