GPG-signed messages show up as a .txt attachment

[austinhuang0131]

Sample: https://github.com/austinhuang0131/barinsta/issues/286

The .asc file is the signature, and most email clients treat it as attachment, so that's fine. However, most email clients will simply show the text content (if the email is not encrypted - in this case, it's not), yet Fire sends a .txt attachment.

Also I used GPGTools 1.6 on Mac