When it comes to the way of saving attachments, our apps are implemented to use 2 steps to reach to the local storage(sharable memory).
It's the same in the Nine apps. Nine and OfficeMail is a secure/reliable business email app. So for example, if we allow users to save all attachments with one step to local storage to be accessed from other apps or systems on your phone, it might bring on a serious problem on your phone's entire system or other whole apps. So we use 2 steps download methods. First, download it to the app's secure internal storage, and if a user want to save it to the local storage of the phone, they can save/save as it there in another step.