Embedded preview image for video files

[baloe]

Is your feature request related to a problem? Please describe.
Hosted nextcloud solutions are substantially cheaper if no ffmpeg is included.
Currently, these services disqualify for me because videos would not have a preview image.
However, I would be happy to manually embed previews in the video files before uploading.

Describe the solution you'd like
Use embedded video preview images such that videos have previews even when there is no ffmpeg.
As a bonus this gives the user control over the preview frame.