I'm running PHP 7.1 artisan serve on Bash on Ubuntu on Windows 10.
Laravel 5.4 Artisan server runs on 8080 port. Problem is that all generated file URL's to storage dir are missing port. URL's are generated using http://glide.thephpleague.com , that uses League\Flysystem\Filesystem
APP_URL as I understand is for CLI commands only, so no use of it in this situation.
Tested the setup on macOS Sierra 10.12 and URL's are generated with port number and files load correctly,