diff --git a/src/mime/mediatype.go b/src/mime/mediatype.go
index 3d480a9d7e7c4..fc6e0d0673d54 100644
--- a/src/mime/mediatype.go
+++ b/src/mime/mediatype.go
@@ -56,7 +56,7 @@ func FormatMediaType(t string, param map[string]string) string {
 
 		b.WriteByte('"')
 		offset := 0
-		for index, character := range value {
+		for index, character := range []byte(value) {
 			if character == '"' || character == '\\' {
 				b.WriteString(value[offset:index])
 				offset = index
diff --git a/src/mime/mediatype_test.go b/src/mime/mediatype_test.go
index 35b311a4a58b1..945a8189e171c 100644
--- a/src/mime/mediatype_test.go
+++ b/src/mime/mediatype_test.go
@@ -481,6 +481,8 @@ var formatTests = []formatTest{
 	{"noslash", map[string]string{"X": "Y"}, "noslash; x=Y"}, // e.g. Content-Disposition values (RFC 2183); issue 11289
 	{"foo bar/baz", nil, ""},
 	{"foo/bar baz", nil, ""},
+	{"attachment", map[string]string{"filename": "ĄĄŽŽČČŠŠ"}, ""},
+	{"attachment", map[string]string{"filename": "ÁÁÊÊÇÇÎÎ"}, ""},
 	{"foo/BAR", nil, "foo/bar"},
 	{"foo/BAR", map[string]string{"X": "Y"}, "foo/bar; x=Y"},
 	{"foo/BAR", map[string]string{"space": "With space"}, `foo/bar; space="With space"`},