kuzmandi Isn't it easy to implement that if a thumbnail exists, it will be used? _files_videoname_preview.jpg or whatever.
Yes, it's definitely possible to implement such a solution.
But it's not ideal of course. It means you manually need to create a screenshot for all videos, name them appropriately, and preferably resize and compress the image properly also. That's why it's clumsy, just because a hosting service doesn't want to offer FFmpeg.
kuzmandi Maybe it would be possible to save the ffmpeg-created preview images to this kind of filenames too (instead of md5 hashing) so that I could run it on my local server (with ffmpeg) and move all to the webserver afterwards.
Of course it's technically possible, but again, a clumsy solution. The reason we store all resized images in _files/cache/images, is because we don't want Files Gallery to affect the users own file system. And it must be stored there encoded, because we can't really store the full path name in the file name. Yes it would be possible to just write all thumbnails directly to the directory, but this then makes a mess of the users own file system, which often isn't used only for Files Gallery.
Some good ideas. I think for now, most likely I will offer optional custom _files_filename.jpg thumbnails in a future release.