An IPhone or IPad.
1) Admin Block > Plugins > Web Services > External services
2) Check Enable mobile web service and Save
3) On your iPhone/iPad download last version of My Mobile app. In the content section of the app check that you can download a folder resource (of course the course must have a folder resource with at least a file in it)
4) In 'Admin Block > Plugins > Web Services > External services' and click the 'Edit' link for 'Moodle mobile web service'
5) Click 'Show advanced' and uncheck 'Can download files'. Click 'Saves changes'.
6) in the mobile app, try to download a folder from a different course that you didn't previously visit in the app. The app should send you an error message.