The inability of the email client to attach arbitrary files was due in WP7 and WP8 to the lack of access to user files - an OS problem.And that's why there must be a file manager. But each time one of us says that, someone comes & tells that it's not needed or that it causes harm to the security of WP devices. I'm fed up of these kinds of people. They think that if they don't need a file manager, no one else does.
This problem is no longer there in WP8.1 since apps can call a file picker. ( http://forums.windowscentral.com/windows-phone-8-1/272545-wp8-1-does-have-file-picker.html )
So if there is still a problem here it is due to the email app, not the OS.
I hope that the Office team works in future on the Outlook mobile apps (Email, Contacts, Tasks, Calendar) because they write serious apps but the current apps are a joke and not business-ready.