Look here for official information on resuming partial downloads: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35
Essentially add a header to the get part with "Range: <ByteToStartAt>-<ByteToStopAt>"
You should be able to omit the ByteToStopAt part if you don't know the Content-Length at the start.
Essentially add a header to the get part with "Range: <ByteToStartAt>-<ByteToStopAt>"
You should be able to omit the ByteToStopAt part if you don't know the Content-Length at the start.