There's a wide array of file managers available for BlackBerry 10 but one of my personal favorites is Ghost Commander due to the fact it allows you to dig a bit deeper into the BlackBerry 10 OS then most other file managers available.
Although it'll never win any awards for being the best looking app out there, it has just been updated with a better looking UI than what it was running before. It now supports API 17 which to normal folks means it looks more like a Jelly Bean app when ran through the Android Player.
On top of that addition, the developer also added a Dropbox client to the mix and the ability for processes to be ran in the background. Overall, a nice little update for anyone looking for a better file manager than what BlackBerry 10 offers out of the box.