ajax_page_state is not taken into account for normal GET requests

Follow-up to #2407195: Move attachment processing to services and per-type response subclasses →

Problem/Motivation

       $ajax_page_state = $this->requestStack->getCurrentRequest()->request->get('ajax_page_state');

is wrong.

Proposed resolution

Should be:

       $ajax_page_state = $this->requestStack->getCurrentRequest()->get('ajax_page_state');

but needs tests coverage.

Remaining tasks

- Fix it
- Write tests
- Commit

User interface changes

- None

API changes

- None

Beta phase evaluation