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A file format or file type is the language (or "encoding") used to express data in a file. Operating systems need to know the particular format(s) of a given file in order to read it, and must therefore record metadata about the file.
This is often accomplished by designating the part of a filename following the last dot as the file extension. In Windows almost all files have extensions as they are the only metadata which Windows recognizes for the purpose, while other platforms such as Linux do not require them although they may still be used in an auxiliary role.