File extensions: the letters that tell you what a file really is

What an extension is

A file extension is the short code after the last dot in a file’s name. It tells the computer which program should open the file.

Documents and images

• .docx, .xlsx, .pptx: Word, Excel and PowerPoint files
• .pdf: documents that look the same everywhere
• .jpg, .png: pictures
• .txt, .csv: plain text and simple data

Containers

• .zip: a compressed folder that can hold other files

Programs and scripts (handle with care)

• .exe, .msi: Windows programs and installers
• .bat, .cmd, .ps1, .js, .vbs: scripts that run commands
• .docm, .xlsm: Office files that contain macros
• .iso, .img: disk images that can contain programs
• .lnk: shortcuts that can launch commands

Why it matters

Attackers disguise programs as documents. A file named “Invoice.pdf.exe” shows as “Invoice.pdf” when extensions are hidden, and may even use a PDF icon. Opening it runs a program.

Show extensions on Windows

In File Explorer, select View, then Show, then File name extensions. On older versions, tick File name extensions on the View tab.

On a Mac, open Finder, then Settings, then Advanced, and tick Show all filename extensions.

A simple rule

Be suspicious of any program or script file that arrives by email or download when you were expecting a document.