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A file extension is an identifier used as a suffix to a name of the computer file in an operating system such as Microsoft Windows. It can be categorized as a type of metadata. A file extension helps the operating system to understand the characteristics of the file, and to some extent, its intended use.
A complete filename includes the name of the file as well as its extension. It is usually three or four characters long, although in rare cases it could be one or two, and it is used as a suffix to the file name. It is often separated from the filename with the help of the dot (.) symbol. Some operating systems include the file extension as part of the file system itself, thus limiting the format and length of the extension. In the case of Windows operating systems, file extensions are usually hidden from users. File extensions can be renamed, however simply renaming a file extension does not necessarily convert one file format to another.
File extensions help the file system and users mainly in two ways. First, it helps in identifying the type of data it holds. Second, the file extension allows the operating system to choose the proper application with which to open a file. This is convenient to users and helps in learning and operating on unfamiliar data files with the help of their file extensions.