File extension

Definition

A file extension (also filename extension or file name extension) is "[a] period and one or more letters at the end of a filename intended to indicate the file’s type."

Overview

In some operating systems (for example Unix) it is optional, while in some others (such as Windows) it is a requirement. Some operating systems limit the length of the extension (such as DOS and OS/2, to three characters) while others (such as Unix) do not. Some operating systems (for example RISC OS) do not use file extensions.

Common file extension names include:

• .bmp — An image file
• .exe — An executable program file
• .gif, .jpeg, .jpg, .tiff, .png — A picture file
• hqx — A file that has been "binhexed" (commonly used for Macintosh archives).
• html, htm — HyperText Mark-up Language (a Web page)
• .mov, .avi, .mpg, .qt — A video file
• .wav, .au, .aif — A sound file
• .xls — A spreadsheet
• .zip — A compressed archive file based on PKzip (commonly used for DOS/Windows)