I tried that, and I get 500 Internal Server Error.
and more lines what I tested :
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

in any case, dosen't worked.

temporarily, I solved problem.
upstream default.folderfile.net {
server folderfile.net;
}
upstream kr.folderfile.net {
server nixneo.com;
}

location / {
proxy_pass http://$geo.folderfile.net;
}

and I added record for default.folderfile.net -> folderfile.net's server IP & kr.folderfile.net -> nixneo.com's IP
so, that image appeared, the link was : http://default.folderfile.net/modules/document/tpl/icons/file.gif


but.. It is incomplete solution.

* now, you can see that site. I opened my server.