I have backed up data, with attachments, in an ArcGIS Online feature layer to a zipped file geodatabase on my local machine. When I test restoring the data back to AGOL with the "Update data" tool the data are restored, but without the attachments. I've checked the contents of the file geodatabase in ArcMap and the data with attachments are there. I've noticed similar results with related tables.
Is this a bug or am I missing something?