About a week before now (22. august) I uploaded a few WEBM files. And the thumbnail images had been false/wrong.
Except one that been around 4MB max. So I changed the other WEBM files to 4MB max and they had the correct thumbnails.
This week (31. august) I uploaded again a few WEBM files with max 4MB file size. And sadly again I got the false/wrong thumbnails again.
I won't reduce the WEBM files to more smaller size (to get the correct thumbnails).
Please, Fix the WEBM 'images' thumbnails.