In the app, go to private files and check that you can upload files using any of the upload methods: photo album, camera, audio, video, file (Android only) and shared files (iOS only). Check that you never go to a new state (new page) when picking the file, instead an action sheet (list of possible options) is displayed in the current page. Now go to an assign that allows files in submissions. Check that you can attach files using any of the methods (online and offline). Please notice that in offline you can only add new submissions, you cannot edit existing ones.