This doesn't need to (and shouldn't) be done in click itself. click already has support for system packages declaring hooks, so some suitable system package that consumes MIME type declarations could declare such a hook, and then click packages would be able to attach to it.
Unfortunately, I don't know where the right package to reassign this to would be. Perhaps you could ask around and try to find that out?
This doesn't need to (and shouldn't) be done in click itself. click already has support for system packages declaring hooks, so some suitable system package that consumes MIME type declarations could declare such a hook, and then click packages would be able to attach to it.
Unfortunately, I don't know where the right package to reassign this to would be. Perhaps you could ask around and try to find that out?