Gmail already blocks the attachment of certain file types in emails for this very same reason, and those with the .js tag at the end will just be the latest added to the existing list of blocked attachments.
Likewise, you can read Encrypt and password protect your Gmail message in a click
If after February 13th rolls around users are still trying to attach .js files to emails before sending, they will be met with an alert in the attachment link that states the file type has been blocked.
There will be a little “help” link which they can click on that will open a popup menu with more details and information why it was blocked.
This information will also be accompanied by other expandable dropdown links which inform the user of why certain email messages with or without attachments get blocked, so they can brush up on other reasons why an email not be allowed to go through if they care to know.
Already blocked file types:
.ADE, .ADP, .BAT, .CHM, .CMD, .COM, .CPL, .EXE, .HTA, .INS, .ISP, .JAR, .JSE, .LIB, .LNK, .MDE, .MSC, .MSP, .MST, .PIF, .SCR, .SCT, .SHB, .SYS, .VB, .VBE, .VBS, .VXD, .WSC, .WSF, .WSH