nginx.conf

location ^~ /attachments/download_zip/ {
  proxy_pass         http://mongrel;
  proxy_redirect     off;
  proxy_set_header   Host               $host;
  proxy_set_header   X-Real-IP          $remote_addr;
  proxy_set_header   X-Forwarded-For    $proxy_add_x_forwarded_for;
  proxy_set_header   X_Forwarded_Proto  $scheme;
  proxy_read_timeout 120;
  proxy_connect_timeout 120;
  post_action @notify_zip
}

location @notify_zip {
  internal;

  # Current request is GET /attachments/download_zip/ so we we have to rewrite it
  rewrite  ^(.*)$  /download_notify_zip  break;

  proxy_pass         http://mongrel;
  proxy_set_header   RateBytes          $body_bytes_sent;
  proxy_set_header   Host               $host;
  proxy_set_header   X-Real-IP          $remote_addr;
  proxy_set_header   X-Forwarded-For    $proxy_add_x_forwarded_for;
  proxy_set_header   X_Forwarded_Proto  $scheme;
  proxy_pass_request_body off;
  proxy_pass_request_headers off;
}