Retrofit WPMS blogs.dir structure

sunrise.php

<?php
/**
 * Sunrise - load very early.
 */

/**
 * If the request is for a file, send it to the legacy ms-files.php for processing.
 *
 * File requests follow the pattern `multisite.com/sitename/files/2022/06/filename.jpg`
 */
function maybe_redirect_ms_files() {
	$path    = parse_url( $_SERVER['REQUEST_URI'], PHP_URL_PATH );
	$is_file = preg_match( '/[\w-]*\/files\/(.*)/', $path, $matches );

	// If this is a file request, send it to ms-files.
	if ( $is_file ) {
		$_GET['file'] = $matches[1];

		require_once ABSPATH . WPINC . '/ms-files.php';
		die;
	}
}

// Wait for all the multisite code to be loaded, then check if we should process this as a file.
add_action( 'ms_loaded', 'maybe_redirect_ms_files' );

wp-config.php

// Load sunrise.php file early.
define( 'SUNRISE', true );