We currently don’t do previews for videos larger than 100MB, because preview generation must happen on the user device due to end-to-end encryption, and it is computationally expensive for large videos. There are ways to solve this, and we will be making improvements on this in the future.
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It has one now. Unless the video is too big, see here for details: https://www.reddit.com/r/ProtonDrive/comments/zb0f9a/comment/izdueud/