We try our best to support all formats with 1SE. But there's some 3rd party or older types that we had to stop supporting due to it only being used by a handful of folks. If your trying to use a video and it's not working out please reach out to us. We'll try our best to get it working.
You can email us here: firstname.lastname@example.org
If possible, please provide a download link for the video so we can take a look at it. Use a service like Dropbox, Drive, or Wetransfer to send a download link. Email clients have an attachment size limit.