ALL isn’t a real MIME type, so we don’t register it for lookup with the other concrete types. It’s a wildcard match that we use for respond_to negotiation internals.
# File actionpack/lib/action_dispatch/http/mime_type.rb, line 88deffetch(type, &block)
returntypeiftype.is_a?(Type)
@lookup_by_extension.fetch(type.to_s, &block)
end