Restoring Thumbnail Previews in macOS Revision as of Sunday, 25 December 2022 at 00:00 UTC
Got generic-looking icons instead of previews for MP4 files and couldn’t figure out what was going on.
Tried Finder -> View -> Show View Options -> Show icon preview to no avail
Then force quit com.apple.quicklook.ThumbnailsAgent
in Activity Monitor and relaunched Finder. All was well.