(n.) 1. Any small program that plugs into a larger one and provides increased functionality. Extensions allow operating systems to control CD-ROM drives. 2. The letters that appear after the dot (.) in the name of an MS-DOS file to classify and identify the file type. Extensions for some of the frequently used multimedia file types are .avi (Video for Windows), .gif (compressed graphics), .jpg (compressed graphics), .mid (MIDI), .mov (QuickTime), .mpg (MPEG video), and .wav (Windows audio). The Macintosh file-naming system uses two hidden extensions called file attributes that are each four letters long. One is for the file type, and the other is for the creator. See table.
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