File storage on Android has been a complicated subject over the years. It started very simply with private folders for each app and a pair of permissions for read and write access to just about everything else. The seeds of change were planted with Honeycomb when Google quietly closed off write access to secondary storage like SD cards, but most people didn't take notice until Google insisted OEMs enforce the same rules in KitKat.
Each year since, new APIs have been introduced to give developers new ways to work with the filesystem or even abstract it out of view while also improving security and maintaining privacy.
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