void onProgressChanged(int uploadPositionInBytes,
Invoked when upload progress has been updated. Please note, that progress events are fired
only in case file upload takes some time, for small files this event may not fired. Also,
there is no guarantee that this method will be invoked when upload has been completed.
When upload has been completed the engine loads a new web page,
so in order to find out when upload has been completed, listen to the
uploadPositionInBytes - - a number of the bytes uploaded
uploadSizeInBytes - - a total number of bytes that should be uploaded