Here’s the setup I built (screenshots attached). The custom state is attached to the repeating group, using type “Files” and set as a list. The workflow tries to add each fileuploader element value using “this list:plus item” logic, but returns an error.
When I tried attaching the custom state to the page element, btw, it wouldn’t allow me to set up a “this list:plus item” value in the workflow. The page element itself isn’t offered as an option when identifying the value.