File sharing between OS X server and iOS device
Looking at https://jamfnation.jamfsoftware.com/discussion.html?id=16976 the key is to configure a 'mac os x server' account, and then enable iCloud Files. That way you can access files on the server just like iCloud files.

Only downside I've found so far, you need to enable iCloud files too, you can't just use file sharing of OS X Server.