diff --git a/src/mime/mediatype.go b/src/mime/mediatype.go
index fc6e0d0673d..05390773a8a 100644
--- a/src/mime/mediatype.go
+++ b/src/mime/mediatype.go
@@ -56,7 +56,8 @@ func FormatMediaType(t string, param map[string]string) string {
 
 		b.WriteByte('"')
 		offset := 0
-		for index, character := range []byte(value) {
+		for index := 0; index < len(value); index++ {
+			character := value[index]
 			if character == '"' || character == '\\' {
 				b.WriteString(value[offset:index])
 				offset = index