● ES File Explorer
ES File Explorer

This is the 2nd App which I install which I install on any phone that I own or manage, as IMHO it is nothing short of  THE "Swiss army" knife of Apps. 
I think it is so good that I would gladly pay for paid/donate version, if there was one.
Here are some of the many things you can do with:
• manage, edit/copy/move files/folders both on your Android device but also on remote locations such as cloud storages, FTP, WebDav, Windows shares on LANs and obviously copy/move/etc... files/folders between those external storages and your Android device.
 It has a "built in" FTP server to remotely manage the files on your phone, letting you copy/move/create/manage files to and from your computer over WiFi.
 You can share/send/receive any files to another Android device over bluetooth or WiFi
 Lastly, it has a "built in" video and music player, a gallery/pictures browser working with local or remote storage and a ChromeCast streamer. As I said, a "Swiss army" knife. And it's free!

Root neededNo but if you do, it will allow you to more easily manage the files in your system and data partitions (having a NANDroid backup though, is recommended before you start touching files in your system partition).