Not sure if anybody in this thread knows about this but Apple solved the file management problem of iOS in iOS 8 with document provider extension + document picker.
As most of you know, apple added iCloud Drive in iOS 8 which allows apps with document picker to have access to other app's iCloud folder and let the app import the files or edit and save back to that same file. But sometimes people don't want the files to be uploaded to iCloud. Some want to use other storage providers like dropbox, onedrive etc and some want their files kept locally only.
For that Apple added Document Provider Extension which just like iCloud Drive, allows any app (cloud storage like dropbox or local storage like goodreader) have its files accessed by other apps which support document picker.
(Example Image from WWDC session below)
So for example; lets say you have 'Goodreader' app where you store and view all kinds of files and documents.
Previously if you want to edit a document stored on goodreader on other apps like numbers for example you would have to use 'open in' to open that file in numbers, edit it, and use 'open in' again to put it back into goodreader. This method is very messy and it creates multiple copies of the same file inside different apps.
In iOS 8 all you have to do is open numbers, select goodreader in document picker and open the file, edit it and save it back to the same file which is inside the goodreader app. Thats it. No copied created and no mess.
So you can choose any app to be your central hub for storing all your files.
For some reasons Apple did not add document picker in mail app so you cannot add attachments directly from the mail app. But since third party apps can, app like gmail can pick up attachments from other apps.
Of course for all of this to work the apps need to implement the extension.