I wrote a post a few months ago about sharing data between iOS apps and app extensions in which I recommended using NSFileCoordinator and NSFilePresenter. But I had to update the post to remove that portion when some helpful people pointed me to Apple Tech Note 2408, which read in part:
When you create a shared container for use by an app extension and its containing app in iOS 8, you are obliged to write to that container in a coordinated manner to avoid data corruption.