Notifies the client of upload progress. |current| denotes the number of
bytes sent so far and |total| is the total size of uploading data (or -1
if chunked upload is enabled). This function will only be called if the
UR_FLAG_REPORT_UPLOAD_PROGRESS flag is set on the request.
Notifies the client of upload progress. |current| denotes the number of bytes sent so far and |total| is the total size of uploading data (or -1 if chunked upload is enabled). This function will only be called if the UR_FLAG_REPORT_UPLOAD_PROGRESS flag is set on the request.