How serve 404 into static file for example index.html or 404.html

mount.mount("/", staticfile::Static::new(Path::new("../assets/index.html")));

I hope it serve the static file when the Path is file. (fn Path::is_file(&self) -> bool)
/ => index.html
/404 => index.html

more :
https://github.com/rackt/react-router/blob/latest/docs/guides/basics/Histories.md#configuring-your-server