# File Extension

*noun*

say it: **“FILE ek-STEN-shun”** **/faɪl ɪkˈstɛnʃən/**

The letters after the last dot in a file name, such as .cs or .png, that tell the computer what kind of file it is.

## What is **File Extension**?

**File Extension** — The letters after the last dot in a file name. They tell Windows, and you, what kind of file it is and which app opens it: `Program.cs` is C# code, `menu.txt` is plain text and `logo.png` is a picture. File Explorer hides them until you turn on **View → Show → File name extensions**.

## In a sentence

- “Windows opened my notes in the wrong app because I saved them with a .cs **file extension** instead of .txt.”

Level: Novice · [More Operating System terms at this level](https://www.ask-codey.com/glossary?category=operating-system&level=1)

Updated: October 28, 2026

Source: https://www.ask-codey.com/glossary/file-extension