Cyberduck is a free FTP client for Windows and MacOS.
When you open the application, click the Open Connection icon and fill in the fields with your credentials.
When you click connect, you’ll be brought to a pane where you can drag-and-drop a file from the Finder.
The upload will start immediately and you will be notified if there are any problems via a popup. Once the transfer is complete, you can check the import page in the admin dashboard for updates.