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)
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