File Formats Wiki
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A filename extension is a suffix to a filename that can be used to identify the format of the file.

For example, files with filenames ending with ".txt" are plain text file, ".rtf" are rich text files, and ".xml" are XML files.

Extensions are not often that accurate in defining a file format. For example, one may rename a file to have an extension of another file format, and the file would be recognized as that other format, but the file may not even be a valid file complying to that format. Other methods are also used to address this, such as associating the file or stream with an Internet media type, or using metadata.

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