Using an external service will not be feasible, because even if you find it, you will need to draw all the videos on it and return the results of the thumbnails.
I recommend that you post your videos somewhere where you can install ffmpeg or some other utility that will create your thumbnails, and then access (link) all the original server location.
EDIT:
Another alternative is to create thumbnails on the client. Can you describe how to create and upload a video?
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