This is so clunky
For reviewables @eviltrout introduced a general mechanism where arbitrary objects could be linked to a reviewable.
Moving to a similar system for uploads would be like moving multiple mountains. It would be a nice mountain to move though cause this current system is pretty fragile.
Easiest workaround you have for now is to create a hidden site setting with a data_type for :upload. Then put it in the site settings table.