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A file extension is the suffix at the end of a computer filename, typically separated by a period (e.g., .txt, .jpg). Its purpose is to indicate the file's format, which allows the operating system to identify the type of data the file contains and which software should be used to open or run it.

How file extensions work

Identification: The extension acts as a label, telling the computer that a file like document.docx is a Microsoft Word document, while photo.jpg is an image.

Program association: When you double-click a file, the operating system uses the extension to determine which program to launch. For example, .mp3 will open an audio player and .xlsx will open a spreadsheet program like Microsoft Excel.

Display options: Many operating systems hide file extensions by default to reduce clutter. You can change this setting in your file explorer to see the full filename, which can offer greater clarity and help with security.

Security risks: Hiding extensions can create a security vulnerability, as malicious files can be misnamed to appear safe. For example, a file named funny.jpg.exe might display as funny.jpg, tricking a user into running a virus.