I am not sure if I understand correctly the issue. The "Receiving metadata from the save handler" example shows how to receive data on the client form the server. As demonstrated in the documentation you should return JSON from the server and use the response event argument in the success event. 
If you wish to receive the data on the server when uploading the file, you should use the event argument data field in the upload event. The data will be sent along with the files as form data.