ES File Explorer was just sorta "there" on my Mi Box, and although it gets things done, it's terrible to navigate (not to mention hated for so many reasons).
I'm aware you can sideload just about anything, but I'm really looking for a file manager that plays nicer with D-PAD input and can access the cloud (Dropbox, GDrive, etc).
X-plore works great has support for Google Drive, and Dropbox and is easy to navigate with a remote. https://play.google.com/store/apps/details?id=com.lonelycatgames.Xplore