Merge pull request #24 from ipfs/make-mimetype-optional

chore: make mimetype optional as it is in go

Author: achingbrain

src/unixfs.proto.js

@@ -20,5 +20,5 @@ module.exports = `message Data {
 }
 
 message Metadata {
-  required string MimeType = 1;
+  optional string MimeType = 1;
 }`