Thumbnails can make it hard to distinguish file types
On a default Fedora 33 installation (Nautilus 3.38), nautilus generates thumbnails for a variety of file types. Unfortunately, these thumbnails can make files of different types harder to distinguish visually than if they were to use generic icons.
Consider the attached screenshot of a folder containing a pdf and a spreadsheet.
As both icons are mostly white rectangles with a smattering of indistinguishable black text, to differentiate the files one really has to read their names, thus defeating the whole purpose of icons.
However, if the thumbnails were to be replaced by the generic icons for PDF and ODS, which I have also included in the screenshot, the two files would be instantly distinguishable. So thumbnails are actually counterproductive in this case.
One solution would be to decorate thumbnails with emblems suggesting file type. For prior art, see this screenshot from OS X
Although many files have thumbnails, file type differences stand out nonetheless. PDF thumbnails have a black spiral on their left border like a spiral notebook binder. Even MP3s and video thumbnails are distinguishable, with MP3 cover art always in a square frame and video clips in a 16:9 rectangle with black borders.
A more near-term fix would be to just generate thumbnails for fewer types of files, such as images and videos. As indicated earlier, the generic icons for other file types are pretty serviceable.