I ran across a blog https://blogs.esri.com/esri/supportcenter/2016/02/24/tips-and-tricks-for-disconnected-editing-with-collector-for-arcgis/ which talks about attachments under number 6. Interested if the second tip listed there has been implemented?
Tip: The next release of Collector will support new offline settings that authors can adjust in ArcGIS Online on the item properties pages of their web maps. These settings will allow map authors to specify what types of information field workers retrieve from the server for both editable and read-only layers. As a result, workers can sync less data, which can decrease the risk of sync failure.