http: allow override of Content-Type for ServeFile

R=bradfitz, bradfitzwork
CC=golang-dev
https://golang.org/cl/4368041

src/pkg/http/fs.go

@@ -134,22 +134,24 @@ func serveFile(w ResponseWriter, r *Request, name string, redirect bool) {
 	size := d.Size
 	code := StatusOK
 
-	// use extension to find content type.
-	ext := filepath.Ext(name)
-	if ctype := mime.TypeByExtension(ext); ctype != "" {
-		w.Header().Set("Content-Type", ctype)
-	} else {
-		// read first chunk to decide between utf-8 text and binary
+	// If Content-Type isn't set, use the file's extension to find it.
+	if w.Header().Get("Content-Type") == "" {
+		ctype := mime.TypeByExtension(filepath.Ext(name))
+		if ctype == "" {
+			// read a chunk to decide between utf-8 text and binary
 			var buf [1024]byte
 			n, _ := io.ReadFull(f, buf[:])
 			b := buf[:n]
 			if isText(b) {
-			w.Header().Set("Content-Type", "text-plain; charset=utf-8")
+				ctype = "text-plain; charset=utf-8"
 			} else {
-			w.Header().Set("Content-Type", "application/octet-stream") // generic binary
+				// generic binary
+				ctype = "application/octet-stream"
 			}
 			f.Seek(0, os.SEEK_SET) // rewind to output whole file
 		}
+		w.Header().Set("Content-Type", ctype)
+	}
 
 	// handle Content-Range header.
 	// TODO(adg): handle multiple ranges

src/pkg/http/fs_test.go

@@ -85,6 +85,30 @@ func TestServeFile(t *testing.T) {
 	}
 }
 
+func TestServeFileContentType(t *testing.T) {
+	const ctype = "icecream/chocolate"
+	override := false
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		if override {
+			w.Header().Set("Content-Type", ctype)
+		}
+		ServeFile(w, r, "testdata/file")
+	}))
+	defer ts.Close()
+	get := func(want string) {
+		resp, _, err := Get(ts.URL)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if h := resp.Header.Get("Content-Type"); h != want {
+			t.Errorf("Content-Type mismatch: got %q, want %q", h, want)
+		}
+	}
+	get("text-plain; charset=utf-8")
+	override = true
+	get(ctype)
+}
+
 func getBody(t *testing.T, req Request) (*Response, []byte) {
 	r, err := DefaultClient.Do(&req)
 	if err != nil {