public function assetUrl(string $asset): string
    {
        $file = $this->resourcesFolder() . $asset;

        // Add the file's modification time to the URL, so we can set long expiry cache headers.
        $hash = filemtime($file);

        return route('module', [
            'module' => $this->name(),
            'action' => 'asset',
            'asset'  => $asset,
            'hash'   => $hash,
        ]);
    }