The right to own and use your files across the platform should be implemented, doesnt matter how. If i have docs in pdf I should be able to use them with email App directly ( dont sing the onedrive song, in corporate environment Cloud file Storage is widely banned)
Quite honestly everyone is not really thinking things through about how WP works. What's needed isn't a file manager, but rather two things:
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File Picker [Coming Soon]
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API's to allow inter-app file transfer between different apps
Currently, Windows Phone operates upon application associations: every file is stored within it's respective app's designated storage. The first of the two required things will allow applications to open files they are not associated with, for instance, in messaging clients, or as mentioned above, eMail clients. Furthermore, the second option on the list would allow the passing of a file from one app to another, for example, sending a PDF through skype. This method would allow for apps to continue to manage their own files while opening up inter-app operability.
Lastly, a "Files" or "Downloads" app should be present that manages a designated "downloads" folder where un-associated files are stored (for example, an exe file downloaded from a business server, or a zip emailed from a coworker, could be stored in "Downloads" for retrieval via USB on a computer.)
A full fledged file manager would in fact be more complicated, decrease security, and require major rewriting of how Windows Phones manages files. The above solution, however. is as simple as adding a few API's. The suggested method would allow one to have more control over their files, while avoiding the clutter and security issues of a full-fledged File Manager.