Once you have enabled the SSH password, you can use SCP command line (Mac, Linux) or WinSCP on Windows to transfer files to or from your Smart Plug.
To transfer files to your plug from another PC on the LAN, you can also run a web server such as Apache2 for Linux, or Civetweb for Windows and place the files in the content folder. Then use the wget command from the Smart Plug command line to transfer the file.