filename extension

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filename extension

(filename extension)
The portion of a filename, following the final point, which indicates the kind of data stored in the file.

Many operating systems use filename extensions, e.g. Unix, VMS, MS-DOS, Microsoft Windows. They are usually from one to three letters (some sad old OSes support no more than three). Examples include "c" for C source code, "ps" for PostScript, "txt" for arbitrary text.

NEXTSTEP and its descendants also use extensions on directories for a similar purpose.

Apart from informing the user what type of content the file holds, filename extensions are typically used to decide which program to launch when a file is "run", e.g. by double-clicking it in a GUI file browser. They are also used by Unix's make to determine how to build one kind of file from another.

Compare: MIME type.

Tony Warr's comprehensive list. Graphics formats.
This article is provided by FOLDOC - Free Online Dictionary of Computing (


(1) See domain extension.

(2) A software add-on. For example, extensions add functionality to Web browsers. See browser extension.

(3) Prior to Mac OS X, an executable module that enhanced the Mac operating system. The Windows counterpart is a "dynamic link library" (see DLL).

(4) Apple enhancements starting with iOS 8 and OS X Version 10.10 that enable apps to share functions in other apps. See "iOS 8" in iOS versions.

(5) A file type that is appended to the end of a file name. All executable programs in the Windows and Mac worlds use extensions: .EXE in Windows; .APP in Mac (see APP file). In the Unix/Linux environment, "executables" do not use an extension, but no matter which environment, "data" files have extensions. For example, a file with a .DOC or .DOCX extension is a Microsoft Word document. A file with a .JPG extension is a JPEG image.

Prior to Windows 95, extensions were limited to three characters. Starting with Windows 95, they can be very large (254-260 characters depending on Windows version); however, they are kept small in practice.

Common Extensions and Exhaustive Lists
In this encyclopedia, more than 500 common file extensions are listed under the terms "extension," followed by their first letter such as extension a, extension b and extension c. However, there are websites that catalog many more, including the most obscure; for example, visit See Win Show file extensions, dangerous extensions and graphics formats. See also domain extension.
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References in periodicals archive ?
This ensures that data are secure unless the user specifically modifies the file extension (e.g., changing .xls to .old, then moving the file to another machine, and finally changing the extension back to .xls).
* File Types--displays the file types associated with a particular file extension and the application to be run if you double-click on the icon.
All infected file attachments use the file extensions PI or.PIF (for example--PASSWORD.PIF), although the files are actually ordinary EXE files.
* The file extensions for particular file formats; and
Other programs allow users to also restrict by file extensions, sharing, for example, all .mp3 and .avi files, but none of the .doc or .pst files.
Naturally the largest chunk of this nearly pocket-sized tome is made up of the words that some of use every day (and a great many more that we don't) while a number of appendices at the back of the book offer lists of textual emoticons, acronyms, top-level domains and common file extensions.
The list of executable file extensions is already so long that it's not realistic to expect every employee to remember them all, and the list keeps growing.
One of the reasons a virus propogates so quickly is because attachment file extensions can be easily hidden from the uninitiated, so when you receive an attachment with a sinister payload, it could only be labelled <attachment.jpeg> as opposed to showing the full file type, IE.
While this made life simpler for people who didn't want to have to learn file extensions, it was confusing for Windows users.
The more singleminded has details about more than 1700 file extensions and directs you to the bulletin board of last resort /ext.htm where you can ask about totally baffling extensions.
The attachment appears to be a simple text file named LIFE--STAGES.TXT, but in reality the file ends with the extension .SHS, which is not visible even if the user's computer is configured to display all file extensions. When the attachment is activated, it introduces changes in the computer and mails out copies of itself to 100 randomly chosen addresses from the user's Microsoft Outlook address book.