diff --git a/src/pkg/mime/type.go b/src/pkg/mime/type.go
index 00cff263ba..d1e403f022 100644
--- a/src/pkg/mime/type.go
+++ b/src/pkg/mime/type.go
@@ -11,19 +11,28 @@ import (
 	"sync"
 )
 
-var mimeTypes = map[string]string{
-	".css":  "text/css; charset=utf-8",
-	".gif":  "image/gif",
-	".htm":  "text/html; charset=utf-8",
-	".html": "text/html; charset=utf-8",
-	".jpg":  "image/jpeg",
-	".js":   "application/x-javascript",
-	".pdf":  "application/pdf",
-	".png":  "image/png",
-	".xml":  "text/xml; charset=utf-8",
-}
+var (
+	mimeLock       sync.RWMutex
+	mimeTypes      = map[string]string{}
+	mimeTypesLower = map[string]string{}
+)
 
-var mimeLock sync.RWMutex
+func init() {
+	mimeTypes := map[string]string{
+		".css":  "text/css; charset=utf-8",
+		".gif":  "image/gif",
+		".htm":  "text/html; charset=utf-8",
+		".html": "text/html; charset=utf-8",
+		".jpg":  "image/jpeg",
+		".js":   "application/x-javascript",
+		".pdf":  "application/pdf",
+		".png":  "image/png",
+		".xml":  "text/xml; charset=utf-8",
+	}
+	for ext, typ := range mimeTypes {
+		AddExtensionType(ext, typ)
+	}
+}
 
 var once sync.Once
 
@@ -31,6 +40,8 @@ var once sync.Once
 // The extension ext should begin with a leading dot, as in ".html".
 // When ext has no associated type, TypeByExtension returns "".
 //
+// Extensions are looked up first case-sensitively, then case-insensitively.
+//
 // The built-in table is small but on unix it is augmented by the local
 // system's mime.types file(s) if available under one or more of these
 // names:
@@ -39,7 +50,7 @@ var once sync.Once
 //   /etc/apache2/mime.types
 //   /etc/apache/mime.types
 //
-// Windows system mime types are extracted from registry.
+// Windows system MIME types are extracted from registry.
 //
 // Text types have the charset parameter set to "utf-8" by default.
 func TypeByExtension(ext string) string {
@@ -47,15 +58,21 @@ func TypeByExtension(ext string) string {
 	mimeLock.RLock()
 	typename := mimeTypes[ext]
 	mimeLock.RUnlock()
+	if typename == "" {
+		lower := strings.ToLower(ext)
+		mimeLock.RLock()
+		typename = mimeTypesLower[lower]
+		mimeLock.RUnlock()
+	}
 	return typename
 }
 
 // AddExtensionType sets the MIME type associated with
-// the extension ext to typ.  The extension should begin with
+// the extension ext to typ. The extension should begin with
 // a leading dot, as in ".html".
 func AddExtensionType(ext, typ string) error {
-	if ext == "" || ext[0] != '.' {
-		return fmt.Errorf(`mime: extension "%s" misses dot`, ext)
+	if !strings.HasPrefix(ext, ".") {
+		return fmt.Errorf(`mime: extension %q misses dot`, ext)
 	}
 	once.Do(initMime)
 	return setExtensionType(ext, typ)
@@ -70,8 +87,11 @@ func setExtensionType(extension, mimeType string) error {
 		param["charset"] = "utf-8"
 		mimeType = FormatMediaType(mimeType, param)
 	}
+	extLower := strings.ToLower(extension)
+
 	mimeLock.Lock()
 	mimeTypes[extension] = mimeType
+	mimeTypesLower[extLower] = mimeType
 	mimeLock.Unlock()
 	return nil
 }
diff --git a/src/pkg/mime/type_plan9.go b/src/pkg/mime/type_plan9.go
index b8f0511ee7..8cbf6777f1 100644
--- a/src/pkg/mime/type_plan9.go
+++ b/src/pkg/mime/type_plan9.go
@@ -48,6 +48,6 @@ func initMimeForTests() map[string]string {
 	return map[string]string{
 		".t1":  "application/test",
 		".t2":  "text/test; charset=utf-8",
-		".png": "image/png",
+		".pNg": "image/png",
 	}
 }
diff --git a/src/pkg/mime/type_test.go b/src/pkg/mime/type_test.go
index 07e1cd5dae..3ec86fbb54 100644
--- a/src/pkg/mime/type_test.go
+++ b/src/pkg/mime/type_test.go
@@ -4,7 +4,9 @@
 
 package mime
 
-import "testing"
+import (
+	"testing"
+)
 
 var typeTests = initMimeForTests()
 
@@ -14,16 +16,20 @@ func TestTypeByExtension(t *testing.T) {
 		if val != want {
 			t.Errorf("TypeByExtension(%q) = %q, want %q", ext, val, want)
 		}
-
 	}
 }
 
 func TestCustomExtension(t *testing.T) {
-	custom := "text/xml; charset=iso-8859-1"
-	if error := AddExtensionType(".xml", custom); error != nil {
+	custom := "test/test; charset=iso-8859-1"
+	if error := AddExtensionType(".tesT", custom); error != nil {
 		t.Fatalf("error %s for AddExtension(%s)", error, custom)
 	}
-	if registered := TypeByExtension(".xml"); registered != custom {
+	// test with same capitalization
+	if registered := TypeByExtension(".tesT"); registered != custom {
+		t.Fatalf("registered %s instead of %s", registered, custom)
+	}
+	// test with different capitalization
+	if registered := TypeByExtension(".Test"); registered != custom {
 		t.Fatalf("registered %s instead of %s", registered, custom)
 	}
 }
diff --git a/src/pkg/mime/type_unix.go b/src/pkg/mime/type_unix.go
index 1d394315a4..3e404cf742 100644
--- a/src/pkg/mime/type_unix.go
+++ b/src/pkg/mime/type_unix.go
@@ -53,7 +53,7 @@ func initMime() {
 func initMimeForTests() map[string]string {
 	typeFiles = []string{"testdata/test.types"}
 	return map[string]string{
-		".t1":  "application/test",
+		".T1":  "application/test",
 		".t2":  "text/test; charset=utf-8",
 		".png": "image/png",
 	}
diff --git a/src/pkg/mime/type_windows.go b/src/pkg/mime/type_windows.go
index 180f948d16..ae758d78b3 100644
--- a/src/pkg/mime/type_windows.go
+++ b/src/pkg/mime/type_windows.go
@@ -58,6 +58,6 @@ func initMime() {
 
 func initMimeForTests() map[string]string {
 	return map[string]string{
-		".png": "image/png",
+		".PnG": "image/png",
 	}
 }