Every file has a name, called the filename.  Filenames are usually in the format filename.ext, where 'ext' is the filename 'extension'.  The extension is used to designate the file "type".

    Almost all information stored in a computer must be in form of a file; and, there are many different types of files: data files, text files , program files, directory files, and many more.  

    Different types of files store different types of information.  For example, program files store programs, whereas text files store text.  Clicking on either of the hyperlinks below will take you to a websites where many of the common file formats are cataloged. You will find the 'extension' followed by a description of the file type...

Webopedia - Data Formats and Their File Extensions

or

Data Formats and Their Suggested File Extensions

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