When I try to import images form a file folder, the folder normally openes in the icon view, which makes it easy to select the needed images.
However, some folders open in the list view, with only file names and attributes visible.
When I move an image from "list-view" folder to "thumbnail-view" folder, I can see the image thumbnail; when I move it back, the thumbnail disappeared. I did not see any differences in the folder settings.
Sorry if I didn't ask the question clearly enough.
I can see the thumbnails in the project panel by selecting "icon view" in the left bottom corner of the panel. My problem is that I can't see the the thumbnails BEFORE they are imported:
File > Import > Look In
displays files inside folders differently for different folders.
BTW, after I posted my question, I noticed the same behavior in Photoshop.