That’s correct – a cloud-hosted app cannot interact with a user’s local network. It can only interact with the network in which the server lives. As in the other thread, you’ll need to use SFTP or an API-based solution like OneDrive, Dropbox, etc.
As for the Java action, files in Mendix are just entities that extend the System.FileDocument entity. So, to pass a document to a Java action, you simply pass the object that represents that file.