Unable to attach files to custom object

[llamafilm]

I'm trying to attach files to a custom object and I can't see a way to do this in the UI.

Netbox Attachments v11.2.3
Netbox v4.5.9
Custom Objects v0.6.0

PLUGINS_CONFIG = {
    'netbox_attachments': {
        'apps': ['dcim', 'netbox_custom_objects'],
        'display_default': 'additional_tab',
    },
}

I expected to see a gray Attachments button at the top of this "Display Calibration" object.

I also tried creating the attachment first, and then assigning it to an object. But I don't see my custom "Display Calibration" object type in the list.