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.
It has one now. Unless the video is too big, see here for details: https://www.reddit.com/r/ProtonDrive/comments/zb0f9a/comment/izdueud/