Why in the world wouldn't you just use NAT/masquerading on the router (which is ideal for making non-routable/private hosts able to open connections to the outside) with scp or even ftp to actually send the file?

ssh tunelling is great and all that, but it's more complicated and may slow things down. The only time I'd recommend it for simple file sending is when (a) you don't have control/intelligence at the router or (b) you're making a quicky VPN using PPP.

c.