If it were not for the frankly baffling response given by an alleged microsoft program manager earlier in this thread, I`d suggest that the app is simply not feature complete.
I suspect someone at Microsoft just plain screwed up with this app and is in CYA mode to save their job. Either a technical error or more likely a fundamental strategic miss of how people use, save and post their photography on the web and covet an App that can pull it all together. Maybe there are legal issues behind the scenes? As a photographer, this app was the coolest thing about Windows 8.
I never did get the system to see my RAW files, including the prior Win8 (even in File Manager), even though it is supposed to (so bought a 3rd party application that did that... but never worked in the Photo App). This App had been the hook to keep me with Windows. I'm no longer hooked.

