Streaming file transfers

Once upon a time, before Matrix, we had a glorious feature of streaming file transfers - where one client would HTTP POST the file using Range headers as the other would HTTP GET it with Range headers also. It worked surprisingly well; good enough to livestream video and do low-latency push-to-talk whilst leaving copies of files on the media repo.

It'd be nice one day to have this in Matrix too. This would mean ensuring that we can send events *before* the media has finished up loading.

Start by reviewing the issue's description of the former Range-header-based HTTP upload and download flow. No files, tests, or entry points are named, so first identify where Matrix media events and media upload completion are specified. Done would require an agreed protocol design that supports sending events before media finishes uploading.

Written by the indexing model from the issue text.