I've always wondered how to do a few of those tricks you see people pull on Discord with embedded media. So, here I've collected the ones I know about, and I'll add to it over time as I figure out how more of them work.
FAKE VIDEO THUMBNAILS
This one's pretty simple, and most likely the one you already know how to do. But just in case, here's how. Open the video you want to use as the actual video in any video editing software, and then insert the image you want to use as the thumbnail as the first few frames. Discord just uses whatever the start of the video is as the thumbnail, so if the first few frames are a completely unrelated image, it'll use that.
'OPEN ORIGINAL' IMAGES
This one involves a file format that I didn't even know existed - APNG. It's a PNG, except... animated. Works similarly to GIFs, but supports 24-bit images and 8-bit transparency. The way this works is similar to the fake video thumbnails. Find an APNG editor, I recommend checking out Ezgif, and make an APNG with the misleading image as the first frame or two, and then the 'payload' image as the rest of the frames, for however long. Discord doesn't know how to animate APNGs (or maybe it just chooses not to, who knows), so it'll only display the first frame as if it were a still image. When you open it in your browser, it animates.
VIDEOS W/ INCREASING LENGTH
Those videos where they start off with a total time of, for example, ten seconds, but as you watch, the end time gets bigger and bigger until it does, finally, eventually end. I know how this one works, but I haven't figured out how to do it myself yet. Essentially, the video's metadata says the video has a length of 0:00:00. So, obviously, Discord doesn't bother showing the end time as anything longer than what it's buffered, since it doesn't actually know how long the video is. But, unfortunately, I have no idea how to edit the length metadata. You're on your own for that one until I figure it out.