Currently, you can’t attach .mp4 videos. Only jpg, jpeg, png, gif, heic, heif, webp are supported. And only gif and webp allow you to attach animations (maybe aPNG is also supported, I’m not sure).
GIF has terrible compression and decoding. webp is better, but the compression ratio is worse than modern video codecs. But creating webp is a real challenge. Any screen recording would be larger in size and worse in quality in .webp if we just attached .mp4.
For .mp4, there are a bunch of compression tools (example for macOS)
Let’s already allow us to attach .mp4 with the size restrictions that are currently set for photos. I want to see the new features of osm.org visually 