You'll want to build an exporter that will work with nodebb-plugin-import: https://github.com/akhoury/nodebb-plugin-import/blob/master/write-my-own-exporter.md
I am just wondering if there is any importer, err, exporter currently doing the job to import attachments/images/files into NodeBB? I haven't found anything mentioned over there, so I wonder if the basic functionality is currently available in the infrastructure or not…?
@JSey there is an open ticket for this here
hopefully will be closing this soon, sorry about the late reply