I don't think you can post videos. When users post links to online videos such as those on YouTube, the video (or at least a preview) is usually added to the auto-generated link box or whatever it is called. If you are asking how to upload a video to the wiki itself rather than a Discusisons/comments/message wall post, you can use Special:Upload. If this is what you are talking about, keep in mind that there are file size and type limits. Depending on your device and/or the length of your video, it may be blocked from upload.
I just tried and, when it comes to Discussions/comments/message walls, it appears you cannot post videos. HybridPix, if you are talking about uploading a video to the wiki itself, then you would need to check the file size and type.