A file extension, or file name extension, is the letters immediately shown after the last period in a file name. For example, the file extension.txt has an extension of .txt. This extension allows ...

In February, the .JS file extension will be added to a list of 31 file types that Gmail already blocks By Lucian Constantin. Jan 26, 2017 11:59 am PST. Image: Martyn Williams/IDG.