SFTP over reverse SSH tunnel?

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http://unix.stackexchange.com – I want to SFTP to a remote computer that is behind NAT. I can't modify the NAT router, so I have set up a reverse SSH tunnel using a middleman server. Here's what my SSH tunnel looks like: laptop--->nat--->middleman<--nat<--desktop The full details are here: Remote desktop over SSH reverse tunnel to replace TeamViewer I am currently using SSH and VNC over this tunnel. How do I use SFTP over the tunnel? I assume the command will look something like sftp://localhost:port SSH login is via key pair only (no password) so we need to consider that in the solution. And I use K (HowTos)