The only way I know of is to upload the video to a hosting site like
www.youtube.com
, and then paste the link in the text of your message. The site will take it from there and load the preview, etc..
Note: I'll move this post to the support section after you've had a chance to see it.
I upload to Photobucket (free version). It resizes the file to a usable size, and after it's there you can click on it to copy the file address, then paste that into your posts.