title: NSURLSession: background upload and then call a service api
body: <p>I was trying to use the new ios7 background transfer api to upload some photos to a server. what it happened now is
  1) we upload the bytes to s3
  2) call a service api to 'complete' the upload</p>
  
  <p>i looked at this doc and it seems background NSURLSession doesn't support 'data' task. does that mean i can't do the step 2 in background after the actual upload is done?</p>