How do you provide an icon for an action extension?

All the extension types use the containing app's icon, with the exception of action extension. The Apple documentation says this:

In iOS, a custom Action extension uses a template image version of its containing app’s icon, which you must provide.

However it does not say how you must provide the icon. It says:

For information on how to add an icon to your app extension, see Creating an Asset Catalog and Adding an App Icon Set or Launch Image Set.

But the links to "Creating an Asset Catalog" and "Adding an App Icon" documentation are links to how to add an icon for an app, they don't contain a section on how to add an icon specifically for an action extension.

The extension target does not contain an app icon section in the XCode general settings (unlike an app), so its icon presumably must be added via an asset catalog. However, even after I added an app icon image set to my extension's asset catalog, the extension's icon is still not appearing.

How am I supposed to add an icon for an action extension? And if it's via its asset catalog then why is that not working when I've added an icon image set to the asset catalog?

In order to get it to work its not enough to add the assets to the asset catalog - its additionally necessary to set the Asset Catalog compiler options of Build Settings section of the extension to specify the name of the app icon set within the asset catalog: