Currently, there are more than two dozen potentially dangerous file types that can’t be used as attachments in Gmail, including .exe, .jar, .sys, .scr, .bat, .com, .vbs and .cmd. Starting with February 13, 2017, .js files will also be added to the list.
Users who attempt to attach these types of files will see a message informing them that the file has been blocked for security reasons. A “Help” link will be provided for people who may want additional information.
For cases where users need to send .js files for legitimate reasons, Google recommends using Drive, Cloud Storage or other file-sharing services.
Google made several security improvements to Gmail in the past year: it enhanced security alerts, it started flagging unauthenticated messages and potentially dangerous URLs, and it disabled support for the RC4 cipher and the SSLv3 protocol.