Malicious emails often attach various forms of executable programs and trick users into running them. These include standard Windows executables (.exe), batch files (.bat), and even JavaScript files (.js). Starting February 13, 2017, Google will not allow JS files to be sent as an attachment, including JS files detected within archives.
Article Link: http://www.androidpolice.com/2017/01/25/gmail-wont-allow-javascript-file-attachments-starting-february-13/