render/download.go
func (e *Engine) Download(ctx context.Context, name string, r io.Reader) Renderer {
	return Download(ctx, name, r)
}
func (e *Engine) Download(ctx context.Context, name string, r io.Reader) Renderer {
	return Download(ctx, name, r)
}
// Download renders a file attachment automatically setting following headers:
//
//   Content-Type
//   Content-Length
//   Content-Disposition
//
// Content-Type is set using mime#TypeByExtension with the filename's extension. Content-Type will default to
// application/octet-stream if using a filename with an unknown extension.