By default windows always comes with file extensions hidden. But this feature is not good for your system security.
Because many times when you download file from internet or some other sources ( CD, PenDrive ), some file comes with virus.
Generally you don’t understand, because this file normally display like .jpg. Bmp .mp3, and its extension is hide (windows always comes with file extensions hidden)
» Read more: Avoid virus installation using Turn on file extension



