How do you explain in a few sentences:
a) why you can open a PDF file with Adobe Reader but not Microsoft PDF Reader?
b) why music players can play OGG and FLAC files but not MP3s?
c) why UC Browser saves every file as an MP3? Why file browsers tell you to append a ".y" or some such at the end of all your files?
d) Why a file browser shows epubs but not mobis?
None of those issues have anything to do with the lack of a file manager. Issue (c) is somewhat related to programmatic file system access, so, in a nutshell:
Every app you install can optionally register a
limited number of file-types that it knows how to work with (like the PDF file extension). Apps that ship with the OS (like XBOX music) do the same. The sum of these file-type registrations constitute the known file-types, meaning the OS recognizes them and knows what to do with them. Apps are only given access to those file types for which they have registered. As far as an app is concerned, no file types beyond those for which it has registered exist on the device. Whenever the OS encounters a file of a known type, it will retrieve that file (from the internet, local file system, wherever) and pass it off to the app that registered for it. If more than one app registered for a specific file-type, the user must choose which app should receive it. Whenever the OS encounters a file of an unknown type, WP has no idea what to do with it, so it is ignored (not downloaded, not visible to the app, etc).
Those few sentences explain issue (c), and many other problems people have with random file access on WP. By registering files of type "*.y" with the OS, and then renaming every file to carry that extension, apps gain the (crippled) ability to deal with files of any arbitrary (unknown) file-type.
The thing is, the introduction of a file manager doesn't necessarily change any of that behaviour! Why should it? If this is one of the problems you want solved (and I would agree it needs solving),
then you're not asking for the right solution! In the very least, you aren't asking for a solution that is guaranteed to solve the problem.
Coming up with a solution to this problem, without discarding all the security principles on which WP is based, is hard. Under the hood, WP and Android are very different, so the same solutions aren't necessarily applicable to both. Ultimately, we just aren't qualified to think up the solutions to these software engineering problems, and that is what a file manager is. It's an assumed solution, which often isn't even related to the problems people expect it to solve. Unfortunately, the community never gets around to discussing what we'd actually want solved, because these threads always get stuck in the worthless pro-/contra- file manager debate.
Instead of wasting time with imaginary solutions, it would be far more interesting to discuss the problems people have (so I do appreciate your list of gripes), which brings me to your other points, just briefly however, since they aren't related to the file manager topic of this thread:
a)
MS PDF reader is known to be quirky. Whatever your exact problem with it is, the arrival of a file manager won't magically fix bugs in MS's PDF reader app.
b)
I have no idea why Windows XP player can't play mp3 files. It is possible to play MP3 files on WP, as demonstrated on MSDN
here.
d)
How am I to know why some unspecified app can handle one file type but not another? Maybe because it didn't register itself for *.mobi files? Possibly the app simply can't deal with such files? Could it be a bug? I don't know. Whatever the problem is, I'm sure that a file manager won't magically enable *.mobi support.