We try our best to support all video formats on 1SE. However, we had to stop supporting some third-party or older types because only a handful of people use them. If you're trying to use a video and it's not working, please reach out to us. We'll do our best to get it working. 

You can contact us through the app's Settings > Contact Support.  

Please send a download link for the video so we can review it. Use a service like iCloud, Drive, Dropbox, or Wetransfer to send a download link. Email clients have an attachment size limit.