> You download a picture in a browser, crop it in a photo editor and attach it to an email. All three apps need access to the same picture. Your backup app needs access to everything.
On Android, each of those three apps would ask you for file system permissions on first launch. Your choices are "full access to user files", "limited access" (usually one directory and all its sub-directories), "full access, but only this time", and "no access".
Both the "save file as" and the "open file" dialog only show directories the app can access, and have a button at the top that reads something like "change storage scope" or "allow more access".
The system even has options where apps can request access to e.g. all photo/video/media directories - the photo editor would probably request only those to begin with.
Also, apps can pretty much never access each others config/keys/etc files - which they never should. If they need to communicate with each other, they're supposed to use interfaces like the Content Provider, Intents or Bound Services.
I think it's pretty well designed.