Attachments added to a feature layer brought to ArcGIS Pro version 3.1.0 or later through a portal connection are inaccessible, and the "MacBinary archive" error is returned in the ArcGIS Field Maps mobile app on iOS devices.
Use an Android device to access the attachments in ArcGIS Field Maps.
Add attachments in ArcGIS Online.
Open the web map or feature layer in the ArcGIS Pro project and not from the portal connection. Add attachments in ArcGIS Pro and overwrite the feature layer.