At the moment the FileMimeTypeTrait uses type hint as class name. The core issue https://www.drupal.org/project/drupal/issues/3579673 made the change but it is not yet in any stable version and also for backwards compatibility with lower core versions the support of ProxyClass should still be there. I suggest using the `Symfony\Component\Mime\MimeTypeGuesserInterface` interface for type hint instead. It will cover both proxy and "normal" class.