File Explorer shows you thumbnails for all files unless it’s a file it doesn’t recognize. Videos almost always have a thumbnail and unless a specific thumbnail is set for it, Windows 10 will use the first frame as the thumbnail. This may, or may not help you identify what the video is. You can set a different thumbnail image for a video and make the thumbnail for it more useful. Here’s how. https://www.addictivetips.com/windows-tips/set-thumbnail-image-for-a-video-on-windows-10/