Fixing video and audio scrubbing

Video and audio uploaded to an app played from the start and nothing else. Pausing or dragging the playhead did nothing, and the preview stayed pinned to the first frame.

A media element only treats a file as seekable once the server proves it can serve part of it on request, which nothing was doing. Every path that serves your files now does, so scrubbing works, and seeking reads just the part it needs instead of the whole file.