For consistency this PR introduces using custom markdown and short upload:// URLs for video and audio uploads, rather than just treating them as links and relying on the oneboxer. The markdown syntax for videos is  and for audio it is .
This is achieved in discourse-markdown-it by modifying the rules for images in mardown-it via md.renderer.rules.image. We return HTML instead of the token when we encounter audio or video after | and the preview renders that HTML. Also when uploading an audio or video file we insert the relevant markdown into the composer.
Martin Brennan
Joffrey JAFFEUX
Guo Xiang Tan
Penar Musaraj