hi woodybwp
agreed, the best option for one-off downloads is definitely a FGDB w/attachments embedded. I believe esri is working on this right now. fwiw, the attachments that are copied to DropBox have the feature Id embedded in the name of the file. also, in the next revision, the geojson backup file also includes the attachment IDs/urls directly in the properties collection.
We got some great feedback and are revamping the Arc2Box service to include some additional features that will help for continuous syncing of feature edits made in ArcGIS Online (via any of it's apps/apis etc). We will still use the AGOL export facilities for shp backup (and the FGDB when that's ready) but the main backup/sync engine now uses another method to continually find changes to the feature service layers (inserts/updates/deletes/attachments). The changes are then exposed as edit feeds that can be monitored by desktop apps (or any app) to keep their local datasources synchronized. Near realtime access to the edit feed also opens up a whole host of other services as well, but more on that coming soon.
cheers
brian