Attachment->user() won't work

[ghost]

Since method Attachment->user() uses config('filer.user.model') to get the related user model class, the published config file should also provide a related entry like

    'user' => [
        'model' => App\User::class,
    ],

or something similar. Otherwise the use of the method will raise an execption

FatalErrorException in Model.php line 765: Class name must be a valid object or a string