this is what i got with spec (not schema):

;; file upload
(s/def ::filename spec/string?)
(s/def ::content-type spec/string?)
(s/def ::size spec/int?)
(s/def ::tempfile #(instance? File %))
(s/def ::file (st/spec {:spec (s/keys :req-un [::filename ::content-type ::size]
                                      :opt-un [::tempfile])
                        :json-schema/type "file"}))

(POST "/file" []
        :summary "post a file"
        :multipart-params [file :- ::file]
        :return ::any?
        :middleware [multipart/wrap-multipart-params]
        (ok (do (println file)
                (dissoc file :tempfile))))

you should be able to add more multipart-params?