Check for media proxy base_url, not Upload base_url

lib/pleroma/plugs/http_security_plug.ex

@@ -114,7 +114,7 @@ defp get_proxy_and_attachment_sources do
       end)
 
     media_proxy_base_url =
-      if Config.get([Pleroma.Upload, :base_url]),
+      if Config.get([:media_proxy, :base_url]),
         do: URI.parse(Config.get([:media_proxy, :base_url])).host
 
     upload_base_url =