Comment 5 for bug 1335524

Enable the "download" flag in playbin for that. You might need to do some work to allow support for download buffering, which could allow you to play and download at the same time.

  flags : Flags to control behaviour
                        flags: readable, writable
                        Flags "GstPlayFlags" Default: 0x00000617, "soft-colorbalance+deinterlace+soft-volume+text+audio+video"
<snip>
                           (0x00000080): download - Attempt progressive download buffering
<snip>

Eventually, you'll want to be able to download using GStreamer, so that you could switch to playing in GStreamer when the user starts playing it mid-download (this is something we'll want in Totem too).