IMPORTANT NOTE
These APIs do not replace the existing v4 APIs yet, and currently only applies to newly uploaded assets still in Draft state (before publishing). Cannot be yet used for Assets that are in Asset Bank and Waiting room.

This step actually starts uploading the file. Send this request to the signed uploadUrl returned by Create upload url. This initializes a resumable (tus) upload. Any file namings have to be base64 encoded/decoded.
The response headers - in particular the Location header - are required for the next call, Upload file chunks.

The {uploadUrl} path parameter is a placeholder for the signed URL from the previous step.