The part of a computer filename that starts with a dot and is followed by three characters is called the extension. It usually tells your computer what kind of file it is, and what program is needed to open it. If, however, your computer meets a file extension it doesn’t understand, you’ll need to do some research to figure it out. Here are two sites that will help: FILExt and File Extensions.
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Feldman, Barbara. "File Extensions." Surfnetkids. Feldman Publishing. 18 Sep. 2007. Web. 23 Sep. 2014. <http://www.surfnetkids.com/tech/276/file-extensions/ >.