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.
If you’re not familiar with web development, JavaScript is a common language used when developing web applications, and JS files are often loaded as part of web pages.
Gmail won’t allow JavaScript file attachments starting February 13 was written by the awesome team at Android Police.