http: fix Content-Type of file extension.

ServeFile() pass empty string to serveFile(). serveFile() should get
file extension via joining root and filename.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4654089

src/pkg/http/fs.go

@@ -222,7 +222,8 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 
 // ServeFile replies to the request with the contents of the named file or directory.
 func ServeFile(w ResponseWriter, r *Request, name string) {
-	serveFile(w, r, Dir(name), "", false)
+	dir, file := filepath.Split(name)
+	serveFile(w, r, Dir(dir), file, false)
 }
 
 type fileHandler struct {

src/pkg/http/fs_test.go

@@ -175,6 +175,21 @@ func TestServeFileContentType(t *testing.T) {
 	get(ctype)
 }
 
+func TestServeFileMimeType(t *testing.T) {
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		ServeFile(w, r, "testdata/style.css")
+	}))
+	defer ts.Close()
+	resp, err := Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "text/css"
+	if h := resp.Header.Get("Content-Type"); h != want {
+		t.Errorf("Content-Type mismatch: got %q, want %q", h, want)
+	}
+}
+
 func TestServeFileWithContentEncoding(t *testing.T) {
 	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
 		w.Header().Set("Content-Encoding", "foo")

src/pkg/http/testdata/style.css

@@ -0,0 +1 @@
+body {}