I just installed the newest version of Dropbox to my laptop to test if this works before buying 3TB and in synced (and locally saved) Dropbox folder the .clip files are missing thumbnail view.
If I copy the same file to my desktop or any other folder outside of Dropbox, thumbnail shows correctly. If I copy it back to Dropbox folder, thumbnail goes missing.
Like said, the folder is set to offline use and stores a local copy.