How to set the HTTP accept header based on the file extension in Nginx

nginx-file-extension-to-accept-header.txt

# Copy the accept header to a variable
set $acceptHeader $http_accept;

# Does the request end in xml or json?
if ($uri ~ ^(.*)\.(xml|json)$) {
  # Create a new accept header...
  set $acceptHeader "application/$2";
  # ...and remove the extension from the path
  rewrite ^(.*)\.(xml|json)$ $1 last;
}

# Set the Accept header for the proxied resource
proxy_set_header Accept $acceptHeader;

# ...continue with normal proxy conf, host and path mapping etc