Unfortunately, the File Transfer connection mode is not available in Viewer for Mac/Linux yet since it's still in the beta stages. Currently, the only connection methods available for Mac/Linux users are Full Control and View Only.
However, the File Transfer connection mode as well as other connection modes will be implemented to Viewer for Mac/Linux at the subsequent beta stages or the final release. You can find the full list of features that are already available in Viewer for Mac/Linux on this page (please see the very first message in the thread). Sorry if this makes our software less useful for you.