Add file response support to wrap-content-type

If wrap-content-type cannot work out the content type from the URI, and
it receives a response with a File as a body, it will attempt to work
out what the content type is from the file extension of the response
body.

ring-core/src/ring/middleware/content_type.clj

@@ -3,6 +3,11 @@
   (:require [ring.util.mime-type :refer [ext-mime-type]]
             [ring.util.response :refer [content-type get-header]]))
 
+(defn- guess-mime-type [{:keys [uri]} {:keys [body]} mime-types]
+  (or (ext-mime-type uri mime-types)
+      (when (instance? java.io.File body)
+        (ext-mime-type (str body) mime-types))))
+
 (defn content-type-response
   "Adds a content-type header to response. See: wrap-content-type."
   {:added "1.2"}
@@ -12,7 +17,7 @@
    (if response
      (if (get-header response "Content-Type")
        response
-       (let [mime-type (ext-mime-type (:uri request) (:mime-types options))]
+       (let [mime-type (guess-mime-type request response (:mime-types options))]
          (content-type response (or mime-type "application/octet-stream")))))))
 
 (defn wrap-content-type

ring-core/test/ring/middleware/test/content_type.clj

@@ -39,7 +39,15 @@
     (let [response {:headers {"CoNteNt-typE" "application/x-overridden"}}
           handler (wrap-content-type (constantly response))]
       (is (= (handler {:uri "/foo/bar.png"})
-             {:headers {"CoNteNt-typE" "application/x-overridden"}})))))
+             {:headers {"CoNteNt-typE" "application/x-overridden"}}))))
+
+  (testing "fallback on response file extension"
+    (let [response {:body (java.io.File. "test/ring/assets/index.html")}
+          handler  (wrap-content-type (constantly response))]
+      (is (= (-> (handler {:uri "/"}) (dissoc :body))
+             {:headers {"Content-Type" "text/html"}}))
+      (is (= (-> (handler {:uri "/index.txt"}) (dissoc :body))
+             {:headers {"Content-Type" "text/plain"}})))))
 
 (deftest wrap-content-type-cps-test
   (testing "response without content-type"