diff --git a/src/mime/type.go b/src/mime/type.go
index c605a94787..89254f5001 100644
--- a/src/mime/type.go
+++ b/src/mime/type.go
@@ -25,7 +25,8 @@ var (
 		".svg":  "image/svg+xml",
 		".xml":  "text/xml; charset=utf-8",
 	}
-	mimeTypes = clone(mimeTypesLower)
+	mimeTypes  = clone(mimeTypesLower)
+	extensions = invert(mimeTypesLower)
 )
 
 func clone(m map[string]string) map[string]string {
@@ -39,6 +40,18 @@ func clone(m map[string]string) map[string]string {
 	return m2
 }
 
+func invert(m map[string]string) map[string][]string {
+	m2 := make(map[string][]string, len(m))
+	for k, v := range m {
+		justType, _, err := ParseMediaType(v)
+		if err != nil {
+			panic(err)
+		}
+		m2[justType] = append(m2[justType], k)
+	}
+	return m2
+}
+
 var once sync.Once // guards initMime
 
 // TypeByExtension returns the MIME type associated with the file extension ext.
@@ -92,6 +105,26 @@ func TypeByExtension(ext string) string {
 	return mimeTypesLower[string(lower)]
 }
 
+// ExtensionsByType returns the extensions known to be associated with the MIME
+// type typ. The returned extensions will each begin with a leading dot, as in
+// ".html". When typ has no associated extensions, ExtensionsByType returns an
+// nil slice.
+func ExtensionsByType(typ string) ([]string, error) {
+	justType, _, err := ParseMediaType(typ)
+	if err != nil {
+		return nil, err
+	}
+
+	once.Do(initMime)
+	mimeLock.RLock()
+	defer mimeLock.RUnlock()
+	s, ok := extensions[justType]
+	if !ok {
+		return nil, nil
+	}
+	return append([]string{}, s...), nil
+}
+
 // AddExtensionType sets the MIME type associated with
 // the extension ext to typ. The extension should begin with
 // a leading dot, as in ".html".
@@ -104,7 +137,7 @@ func AddExtensionType(ext, typ string) error {
 }
 
 func setExtensionType(extension, mimeType string) error {
-	_, param, err := ParseMediaType(mimeType)
+	justType, param, err := ParseMediaType(mimeType)
 	if err != nil {
 		return err
 	}
@@ -115,8 +148,14 @@ func setExtensionType(extension, mimeType string) error {
 	extLower := strings.ToLower(extension)
 
 	mimeLock.Lock()
+	defer mimeLock.Unlock()
 	mimeTypes[extension] = mimeType
 	mimeTypesLower[extLower] = mimeType
-	mimeLock.Unlock()
+	for _, v := range extensions[justType] {
+		if v == extLower {
+			return nil
+		}
+	}
+	extensions[justType] = append(extensions[justType], extLower)
 	return nil
 }
diff --git a/src/mime/type_test.go b/src/mime/type_test.go
index d2d254ae9a..dabb585e21 100644
--- a/src/mime/type_test.go
+++ b/src/mime/type_test.go
@@ -5,6 +5,9 @@
 package mime
 
 import (
+	"reflect"
+	"sort"
+	"strings"
 	"testing"
 )
 
@@ -43,6 +46,59 @@ func TestTypeByExtensionCase(t *testing.T) {
 	}
 }
 
+func TestExtensionsByType(t *testing.T) {
+	for want, typ := range typeTests {
+		val, err := ExtensionsByType(typ)
+		if err != nil {
+			t.Errorf("error %s for ExtensionsByType(%q)", err, typ)
+			continue
+		}
+		if len(val) != 1 {
+			t.Errorf("ExtensionsByType(%q) = %v; expected exactly 1 entry", typ, val)
+			continue
+		}
+		// We always expect lower case, test data includes upper-case.
+		want = strings.ToLower(want)
+		if val[0] != want {
+			t.Errorf("ExtensionsByType(%q) = %q, want %q", typ, val[0], want)
+		}
+	}
+}
+
+func TestExtensionsByTypeMultiple(t *testing.T) {
+	const typ = "text/html"
+	exts, err := ExtensionsByType(typ)
+	if err != nil {
+		t.Fatalf("ExtensionsByType(%q) error: %v", typ, err)
+	}
+	sort.Strings(exts)
+	if want := []string{".htm", ".html"}; !reflect.DeepEqual(exts, want) {
+		t.Errorf("ExtensionsByType(%q) = %v; want %v", typ, exts, want)
+	}
+}
+
+func TestExtensionsByTypeNoDuplicates(t *testing.T) {
+	const (
+		typ = "text/html"
+		ext = ".html"
+	)
+	AddExtensionType(ext, typ)
+	AddExtensionType(ext, typ)
+	exts, err := ExtensionsByType(typ)
+	if err != nil {
+		t.Fatalf("ExtensionsByType(%q) error: %v", typ, err)
+	}
+	count := 0
+	for _, v := range exts {
+		if v == ext {
+			count++
+		}
+	}
+	if count != 1 {
+		t.Errorf("ExtensionsByType(%q) = %v; want %v once", typ, exts, ext)
+	}
+}
+
 func TestLookupMallocs(t *testing.T) {
 	n := testing.AllocsPerRun(10000, func() {
 		TypeByExtension(".html")