Attachments that are published with a feature class from ArcMap to hosted services are not accessible in ArcGIS Online. When the feature service is added to and viewed in the Map Viewer, a feature's pop-up reads 'No Attachments Found', even though the attachments were successfully published from ArcMap. This behavior is unexpected.
The Object ID field of the feature class used for the feature service is named something other than 'OBJECTID'. For example, the field may be named 'OBJECTID_1' after various manipulations of the dataset. When setting and running Add Attachments to a feature class, the Join Field can be an ObjectID field or any other identifying attribute. This problem occurs when using either option for the Join Field, if the indexed ObjectID field is named anything other than 'OBJECTID'.