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How do I know what programs open which file extensions?

How can I find out what programs are supposed to open files of certain extension? For example, where can I go to get a definition of a .DAT file, and which program was originally used to open it?

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The most comprehensive source for file extension definitions is the File Extension Source, where you can search by the three-letter extension. Some file extensions (like .DAT) are fairly common, however, and are shared by a number of applications.

This was first published in September 2003

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