Return a file server upload URL for an upload link token.
After getting the upload URL, you can upload files to the shared folder; for more info, please see Upload File.

• No account token is needed; anyone with the upload link token can call this endpoint.
• Password-protected upload links require a browser session that has already passed the password check (otherwise 403).
• Expired links return 404. The link creator must (still) have read-write permission on the upload folder.