Solution for syncing directory hierarchy of images from Mac to iOS? I'm a photographer and need to keep an updated set of portfolio images synced from my Mac to my iOS devices and available for offline viewing. I have around 300 images totalling ~150MB)

Right now the directory that I want to sync looks like this on my Mac:

Portfolio-2015/
 Project 01/
   Car.jpg
   Boat.jpg
   Bus
 Project 02/
   Zebra.jpg
   Lion.jpg
 ..x 150


I've found several solutions that accommodate the syncing part of this problem but for some reason none of them them allow you to mark a folder (and its hierarchy) to work off-line on the iOS device.

So far I've tried Google Drive, Dropbox and OwnCloud. All of these have iOS clients that allow you to 'favorite' individual files for off-line access but none of them allow you 'favorite' a folder.

Does anyone know of a solution that will work for me?
You could use Box. 

To my knowledge, they used to offer this feature.