Chinese character in Node Attachment results in error encoded base64 for Github extension

[Haruki Kirigaya]

Hi, there.

I entered some Chinese characters along with a list of urls in the node attachment of a node.
There's no problem if I export the mindmap. But if I save it to Github, I receive a 422 error as follows.

PUT https://api.github.com/repos/zxteloiv/mindmup_storage/contents/20150427.mup

{
  "message": "content is not valid Base64",
  "documentation_url": "https://developer.github.com/v3/repos/contents/"
}

Parameters for this PUT action is something like

{"content":"ewogICJ0aXRsZ...    ","message":"new sections added","sha":"61fc46b75f54a66c9ccfcb72ee3bd0a18afc60d8","branch":"master"}

And I copied out the content field to any online base64 decoding service, I get a stripped json string

     "title": "learning essays",
      "id": 60,
      "attr": {
        "attachment": {
          "contentType": "text/html",
          "content": "�֖Ｑȹё(ݝܹa݌ȀĐ�Ѽ͍Ĺѵȹё(ݝܹa݌ȀĐ�Ѽ͍ȹѵȹё(ݝܹa݌ȀĐ

My environment is

• OSX Yosemite 10.10.3
• Firefox 37.0.2

I hope this problem could be solved, then I can type in things more freely :)
Thank you!