diff --git a/src/pkg/net/http/fs.go b/src/pkg/net/http/fs.go
index e7bcefed15..8b32ca1d0e 100644
--- a/src/pkg/net/http/fs.go
+++ b/src/pkg/net/http/fs.go
@@ -140,9 +140,11 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 
 	code := StatusOK
 
-	// If Content-Type isn't set, use the file's extension to find it.
-	ctype := w.Header().Get("Content-Type")
-	if ctype == "" {
+	// If Content-Type isn't set, use the file's extension to find it, but
+	// if the Content-Type is unset explicitly, do not sniff the type.
+	ctypes, haveType := w.Header()["Content-Type"]
+	var ctype string
+	if !haveType {
 		ctype = mime.TypeByExtension(filepath.Ext(name))
 		if ctype == "" {
 			// read a chunk to decide between utf-8 text and binary
@@ -156,6 +158,8 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 			}
 		}
 		w.Header().Set("Content-Type", ctype)
+	} else if len(ctypes) > 0 {
+		ctype = ctypes[0]
 	}
 
 	size, err := sizeFunc()
diff --git a/src/pkg/net/http/fs_test.go b/src/pkg/net/http/fs_test.go
index 125d8b438d..ae54edf0cf 100644
--- a/src/pkg/net/http/fs_test.go
+++ b/src/pkg/net/http/fs_test.go
@@ -20,6 +20,7 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
+	"reflect"
 	"regexp"
 	"runtime"
 	"strconv"
@@ -319,24 +320,29 @@ func TestServeFileContentType(t *testing.T) {
 	defer afterTest(t)
 	const ctype = "icecream/chocolate"
 	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
-		if r.FormValue("override") == "1" {
+		switch r.FormValue("override") {
+		case "1":
 			w.Header().Set("Content-Type", ctype)
+		case "2":
+			// Explicitly inhibit sniffing.
+			w.Header()["Content-Type"] = []string{}
 		}
 		ServeFile(w, r, "testdata/file")
 	}))
 	defer ts.Close()
-	get := func(override, want string) {
+	get := func(override string, want []string) {
 		resp, err := Get(ts.URL + "?override=" + override)
 		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)
+		if h := resp.Header["Content-Type"]; !reflect.DeepEqual(h, want) {
+			t.Errorf("Content-Type mismatch: got %v, want %v", h, want)
 		}
 		resp.Body.Close()
 	}
-	get("0", "text/plain; charset=utf-8")
-	get("1", ctype)
+	get("0", []string{"text/plain; charset=utf-8"})
+	get("1", []string{ctype})
+	get("2", nil)
 }
 
 func TestServeFileMimeType(t *testing.T) {
diff --git a/src/pkg/net/http/response_test.go b/src/pkg/net/http/response_test.go
index 181937a782..5044306a87 100644
--- a/src/pkg/net/http/response_test.go
+++ b/src/pkg/net/http/response_test.go
@@ -614,3 +614,16 @@ func TestResponseContentLengthShortBody(t *testing.T) {
 		t.Errorf("io.Copy error = %#v; want io.ErrUnexpectedEOF", err)
 	}
 }
+
+func TestNeedsSniff(t *testing.T) {
+	// needsSniff returns true with an empty response.
+	r := &response{}
+	if got, want := r.needsSniff(), true; got != want {
+		t.Errorf("needsSniff = %t; want %t", got, want)
+	}
+	// needsSniff returns false when Content-Type = nil.
+	r.handlerHeader = Header{"Content-Type": nil}
+	if got, want := r.needsSniff(), false; got != want {
+		t.Errorf("needsSniff empty Content-Type = %t; want %t", got, want)
+	}
+}
diff --git a/src/pkg/net/http/server.go b/src/pkg/net/http/server.go
index cc0b4e237b..0e46863d5a 100644
--- a/src/pkg/net/http/server.go
+++ b/src/pkg/net/http/server.go
@@ -341,7 +341,8 @@ func (w *response) requestTooLarge() {
 
 // needsSniff reports whether a Content-Type still needs to be sniffed.
 func (w *response) needsSniff() bool {
-	return !w.cw.wroteHeader && w.handlerHeader.Get("Content-Type") == "" && w.written < sniffLen
+	_, haveType := w.handlerHeader["Content-Type"]
+	return !w.cw.wroteHeader && !haveType && w.written < sniffLen
 }
 
 // writerOnly hides an io.Writer value's optional ReadFrom method