Google Photo's is basically the Gallery App.
As for regular files, if you go into the Settings App, then tap Storage and then Explore at the bottom it brings up a decent enough file explorer that should hopefully do the trick. If not, you could probably find a 3rd party app about the Google Play store.
I did a search and found what looks like a good one by Asus. Its for their ZenPhones with the ZenUI but it seems anyone can download and install it for their phones regardless. If you want to take a look, here's a link. Its got good ratings and whatnot, so it should do. If not, well, there's plenty more.
Hope this helps. Good luck.