Serves a file to a video element with HTTP range requests, including a file that is still being written. Any file: a fragmented MP4 a playlist from Fmp4.playlist/3 points at, a plain MP4, a Matroska release.

A browser plays a plain MP4 by asking for byte ranges (Safari starts with bytes=0-1 to learn the size), so a file has to answer 206 with Content-Range. A file still growing (growing?: true) has no total yet: its ranges are answered with * for the total, a piece at a time so the browser asks again as it fills, and a request past what exists so far waits for the writer to catch up before giving up with 416.

def media(conn, %{"name" => name}) do
  Fmp4.RangeFile.send(conn, path(name), "video/mp4", growing?: not finished?(name))
end