Defines Multisite upload constants.

Exists for backward compatibility with legacy file-serving through
wp-includes/ms-files.php (wp-content/blogs.php in MU).

function ms_upload_constants() {
	// This filter is attached in ms-default-filters.php but that file is not included during SHORTINIT.
	add_filter( 'default_site_option_ms_files_rewriting', '__return_true' );

	if ( ! get_site_option( 'ms_files_rewriting' ) ) {
		return;
	}

	// Base uploads dir relative to ABSPATH.
	if ( ! defined( 'UPLOADBLOGSDIR' ) ) {
		define( 'UPLOADBLOGSDIR', 'wp-content/blogs.dir' );
	}

	// Note, the main site in a post-MU network uses wp-content/uploads.
	// This is handled in wp_upload_dir() by ignoring UPLOADS for this case.
	if ( ! defined( 'UPLOADS' ) ) {
		$site_id = get_current_blog_id();

		define( 'UPLOADS', UPLOADBLOGSDIR . '/' . $site_id . '/files/' );

		// Uploads dir relative to ABSPATH.
		if ( 'wp-content/blogs.dir' === UPLOADBLOGSDIR && ! defined( 'BLOGUPLOADDIR' ) ) {
			define( 'BLOGUPLOADDIR', WP_CONTENT_DIR . '/blogs.dir/' . $site_id . '/files/' );
		}
	}
}