WP doesn't need a file manager per se- there are plenty of file manager apps already.
What WP needs is to have real actual read access to a fourth shared storage area. Pictures hub, music hub, office hub are all accessible by apps indirectly- only by proxy of the system which has pathetic support for file types, and apps cannot extend the system with this model.
Moliplayer is an example of what apps could do if they have actual direct read access- moliplayer, with OEM powers, can directly bypass the system to read files, and thus it can successfully play a video file renamed to sample.mkv.mp3, where as the system just says "this is an invalid audio file". I explained this to demonstrate how the system method of reading the library is inherently restricted to supported file types, and thus cannot be extended.
The reason for the fourth shared storage area is so that any app can export data that can be imported to any other app. Video downloads from YouTube played in any player, file downloads, backing up game saves, there would be a place to actually email files from and save files from emails. Apps could optionally use it by default, and if "use SD card to store files and media" is selected, those of us with 64GB SD cards and 8GB internal storage would have a means of actually making use of the SD while offloading large caches/saved videos or downloads or etc from internal space.
This also solves the issue of opening files in other apps doubling the used storage space. Currently, if you download a 1GB video from the torrent app, and open it with moliplayer via the system's "open with", what happens? The video gets *copied* from the torrent app's isolated store to moliplayer's isolated store. That video a) takes 30s to copy, so there's a huge delay, b) now exists twice on your phone in two separate almost hidden places. 1GB videos need 2GB free to be played. 5GB videos need 10GB free to be played. That is... So stupid. If you could download to and open from a shared storage area- problem solved.
If there were an official utility to import export all isolated store data to/from the shared store area, apps would even need to manually add support for it
File manager and file manager apps are still largely unnecessary under this model, since the shared storage area would be accessible by any app the same way pictures are accessible by any app.