If you are new to #mastodon, migrating from #twitter and you love to post videos, please consider posting your video over at a hosting site such as Youtube or Vimeo (others exist). Then post the link, it should show and play in your Toot.
It saves your server from having to store and stream huge quantities of extra data.
I'm going to add into here that federated hosting exists in the form of #peertube as well which is worth a look at. https://joinpeertube.org/