Computers use file extensions to determine a file's type. The file extension is found after the name of the file and after the period. In the example filename.zip, zip is the file extension.
Purpose
The purpose of the file extension is to identify the type of file. The computer uses this information to open the file with the correct software application.
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Exceptions
Some files do not have extensions. These files are commonly seen in Mac- and Unix-based operating systems.
System Files
Files come in many different varieties, and there are thus many types of file extension. File extensions such as .ini and .dll denote files that contain information the computer uses. These types of files are generally not opened by computer users.
Image and Music Files
Common files that are opened by computer users are image and music files. Examples of image file extensions include .jpg, .gif, .bmp and .png. Examples of music file extensions include .mp3, .mp4, .ra, .wma and .wav.
Other Types
Software executable files have the .exe file extension. Compressed files have the .zip file extension. Depending on the word processor used, a document could have the extension .doc, .docx, .txt or .rtf.