The popup will display the video without upscaling it, but it won't create and display any "preview image" for you. The popup works from a plain text link ... if you want a preview-image as the link, you would first have to create the preview image in JPG format yourself, because X3 cannot create preview images from videos. Anyway, the popup would work like this:
<a href="/root/path/to/video.mp4" data-popup>Open video</a>
The only reason I mentioned popup in the first place, is because it wasn't clear in your initial post where the video was embedded. If you wanted to create a preview image as the link, it would look like this:
<a href="/root/path/to/video.mp4" data-popup><img src="/root/path/to/preview.jpg"></a>