add a default File Browser

I think online File Storage App is fine, such as[Dropbox](https://itunes.apple.com/cn/app/dropbox/id327630330?l=en&mt=8). Alternatively, if someone like me, living in China, I would like to use [Baidu Cloud](https://itunes.apple.com/cn/app/bai-du-yun/id547166701?l=en&mt=8).

Mostly, I'm also upload some database file from app to my computer.