Returns a floating-point value between 0.0 and 1.0, indicating the progress of uploading body data to the server.
If the UnityWebRequest is complete (either a success or a system error), this property will always return 1. If the UnityWebRequest is still communicating with the remote server, and uploadHandler is
null, this property will return zero. If Send has not yet been called, this property will return -1.