chore: make mimetype optional as it is in go

achingbrain · achingbrain · commit 9e0ec8db18cc · 2018-05-14T13:55:01.000+01:00
diff --git a/src/unixfs.proto.js b/src/unixfs.proto.js
@@ -20,5 +20,5 @@ module.exports = `message Data {
 }
 
 message Metadata {
-  required string MimeType = 1;
+  optional string MimeType = 1;
 }`
