Don't add .png and .jpg there. (At best it will do nothing, and at worst it will break the built-in viewer for those formats and replace it with something awful for viewing them, like an embedded web browser.)
If thumbnails are missing from the infotips when you hover over things, you can add them under Settings > File Types. You'll probably need to edit the Info Tip under All Files and under the Images file type group.
Sorry, issue is still exists.
(btw Preferences / File Displays / Options / Enable file InfoTips in - that is tagged)
I have discovered that for photos it doesn't work (no popup) but for screenshot it does.
It doesn't matter what what camera trademark (Canon, Nikon, Sony): No 'preview' when hovering over them.
In the past it worked flawlessly.
Went into the Preferences->Viewer Plugins (here starts a bit of a slippery road for me) and checked MultiView->Configure
and noticed it was linked to a .dll that does not exist (yet).
The only multiview.dll exists in a subfolder of opus itself.
Now, I don't know if for this, i.e. preview when hovering over the files, the multiview.dll is required...
That said, it was impossible to select that folder clicking on [Configure].
The [OK] button remained greyed out.
I even updated the multiview.oxc (then Opus File Exit and restart to load it).
There are a 1000 posts about this multiview file. I don't want to pay and install 3rd party software (except X1).
btw the X1 viewer folder does not contain a file named multiview.