Recent security updates to Google’s mail services mean that Gmail and Gsuite accounts are no longer allowed to attach files with the .js extension (this includes zip files which contain .js files).
This is in addition to the following file extensions which were already banned:
File types you can’t include as attachments
.ADE, .ADP, .BAT, .CHM, .CMD, .COM, .CPL, .EXE, .HTA, .INS, .ISP, .JAR, .JS(NEW)
, .JSE, .LIB, .LNK, .MDE, .MSC, .MSI, .MSP, .MST, .NSH .PIF, .SCR, .SCT, .SHB, .SYS, .VB, .VBE, .VBS, .VXD, .WSC, .WSF, .WSH
This obviously can raise issues, for example if you’re emailing a plugin or theme to a client or similar!