When I select "Open folder" as the action for photos and plug in my camera,
the folder only opens if I additionally check the "Browse media" checkbox.

This is confusing, imo. An explicit choice of "Open folder" should work, independently of the "Browse media" checkbox.

The problem is that autorun_guessed_content_type_callback calls autorun_open_folder_for_mount to open the folder, and makes no difference between explicit and fallback. In the explicit case, we should directly open the window, and not go via autorun_open_folder_for_mount, since that function checks the "Browse media" setting.