OK. I could live with this workaround. But things aren't going so well yet.
Video file: video.mp4
Directory: videos
thumbnail: _files_thumb_video.jpg
Dimensions: 1200x700
Then I cleared the browser cache and used a different browser. I also tried it on another PC, but the thumbnail is not displayed.
This works quite well for the Thumbnails folder.
but only if it is the only thumbnail file. Along with the _files_thumb_video.jpg, the file _filespreview.jpg is ignored. But that's not important for now. It's nice when it works with the video thumbnails...