I found that this call to `sendSynchronousRequest` was blocking indefinitely when called from a thread with with `IOPOL_THROTTLE` set (https://developer.apple.com/library/mac/documentation/Darwin/Reference/Manpages/man3/getiopolicy_np.3.html). I also had other uploads in flight at the same time. I didn't take too long to investigate _why_, but replacing the call with `NSData dataWithContentsOfURL` fixed the problem for me.