NAUTILUS FILE MANAGER(UBUNTU DEFAULT FILE MANAGER) THUMBNAIL ISSUE FIXED!!!
STEPS:-
1. Go to prefernce in Nautilus. And in the thumbnails section change to all files and changes to files under 4800MB.
2. Open Terminal (ctrl+alt+T) and type:- sudo apt install ffmpegthumbnailer. Then press 'Enter' and 'Y'.
3. Close Terminal and Open Nautilus. Press ctrl+H. open .cache folder and inside it open Thumbnails folder. Delete everything inside it.
4. Now close Nautilus and open terminal and type:- nautilus -q.