Windows Phone File Manager to come with Windows Phone 8.1
Since the current version of Windows Phone doesn’t include a File Manager, it is likely to be tested internally and included later as a part of the next OS update. We already have the official changelog of GDR3 (Update 3) and the File Manager is not among the new features, so it could arrive with the next major OS update dubbed Windows Phone 8.1 (a.k.a Blue) in 2014. Another possible option for Microsoft is to add the Windows File Manager as a stand-alone app in the Windows Phone Store.
Source: My Nokia Blog