Although it's not as good as several third party file manager apps you can find on the Google Play Store, it turns out Google surprised us with a "hidden" file manager app built-in to Android 6.0 Marshmallow. You have to wonder why they haven't included anything like this until now (and why they tucked it away without any fanfare), but it's better late than never.
To unlock this "secret" feature, you only need to go into Settings, then go to Storage and USB, and then tap on Explore after scrolling to the bottom. It offers a "list view," a "grid view," and some basic sorting, searching and filtering. There's not much else, but it gets the job done at a basic level.
