Where did you get this file from? .dat files aren't restricted to one type of media or use. It simply just a data file.
Unless you know where the file came from or what it's purpose is then it's just a guessing game.
Most .dat files hold basic text information. Try opening it in notepad. If text appears and the file's intention is to be read, then you're done. If a whole bunch of nonsensical characters, then it's getting difficult.
Next step would be to try opening it in a media player, like Windows Media Player. It might be an audio file or video file with the extension disguised. In fact it may be any file with the extension disguised (that is, the .dat in place or for example, .mp3 or .mpg).
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