Chrome Version       : 53.0.2785.143
OS Version: OS X 10.11.6
URLs (if applicable) : http://storage.googleapis.com/bucketfot.appspot.com/✰.txt
http://storage.googleapis.com/bucketfot.appspot.com/✰✰.txt
http://storage.googleapis.com/bucketfot.appspot.com/star
http://storage.googleapis.com/bucketfot.appspot.com/tahoe.jpg
Other browsers tested:
  Add OK or FAIL after other browsers where you have tested this issue:
     Safari 10: OK

What steps will reproduce the problem?
1. Go to the URLs above. All of them have "Content-Disposition: filename*=utf-8''%E2%9C%B0.txt"
2. Command-S to save the file

What is the expected result?
I expect to get a dialog to save ✰.txt (or ✰) for all of them.

What happens instead of that?
✰.txt gets a prompt to save as %E2%9C%B0.txt
✰✰.txt --> ✰.txt
star --> star.txt
tahoe.jpg --> ✰.txt

Please provide any additional information below. 
✰.txt and star have Content-Type: text/plain
✰✰.txt and tahoe.jpg have Content-Type: image/jpeg

UserAgentString: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.143 Safari/537.36