Thumbnails not showing in video in Ubuntu 18.04

Install FFMPEG Thumbnailer-

sudo apt install ffmpegthumbnailer

Then close and reopen file manager. Thumbnails for most video files should generate now.

However, if it's still not working then open file manager, go to your home folder if you're not already there (Home folder is usually the folder which contains Downloads, Documents, Photos etc folders).

Press Ctrl+H, enter into the folder named .cache. Then enter in the folder thumbnails.
Delete everything in there. Restart your PC.

I have tried HattinGokbori87's solution but it failed, and I found another way to solve it.

First I installed the HattinGokbori87's suggested package like this:

sudo apt install ffmpegthumbnailer

Then I restarted the computer but I still could not see the video's thumbnails in the file manager, so I tried to install another package like this:

sudo apt install gstreamer1.0-libav

Then cleaned up the directory like this:

rm -r ~/.cache/thumbnails/fail

Re-opened the Ubuntu file manager, it works!

Refer to this article if your Ubuntu 18.04 thumbnails still do not work.

Update: For Ubuntu 20.04 this method still applies (I just upgraded my computer operating system from Ubuntu 19.10 to Ubuntu 20.04).