Filename extension

The file name extension (english filename extension), also referred to as the file extension, file extension or file suffix is the last part of a file name, and is usually separated by a dot ( the dot itself is not considered part of the extension). The file extension is often used to make the format of a file recognizable without having to read the file before.

Example: name.txt features a simple text file.

There are no standards body for file extensions. Therefore, it may happen that one and the same file name extension for file types is used.

It is not advisable to change the file extension by simply renaming, for example, from data.txt in DATEN.ZIP. Simple text characters in this example - The file contents, so the actual data, thereby remain unchanged. To change the file format should you convert a file.


Some operating systems and also some individual programs are not able to recognize the type of a file without suffix. Among some popular operating systems, especially Windows and OpenVMS are file extensions associated with certain applications ( file associations ). If you activate a file in a file manager, it will be opened with the associated program.

Other operating systems, such as Mac OS, AmigaOS, UNIX or have additional mechanisms to determine the file format or intended use of a file and use the file extension part for a more precise determination of the size or other purposes ( for example for the version or platform specification for libraries). Sometimes a combination of both approaches is used; For example, the graphical Linux interface KDE relies first on the file extension; However, these missing or is it in the system is unknown, an attempt is made based on the contents of the file to identify the type.

However, possible because of the file extension not be determined with certainty whether a file is actually in the specified file format. This is only possible if the file is opened and the contents, for example, the information in the file header ( magic number ) or characteristic strings are evaluated. However, not every file has a header, for example, have simple text files (. Txt) no special markings, but the content usually starts with the first byte.

The actually more advantageous labeling of file type into separately stored file metadata (a form of out-of- band signaling - an example is Mac OS called, the file types and the program to open stores in a special file space [ so-called resource forks ] ) lost with the dominance of Microsoft operating systems in wide use in practical importance.


In Windows systems is pre-set to the Windows Explorer hides all file extensions known to the system. This fact is exploited by various malicious programs, before the ending of an executable file, a harmless extension is inserted. Thus, for example, from the Trojan horse called Bild01.exe a Bild01.jpeg.exe. The user sees only Bild01.jpeg, so a seemingly harmless image file. However, a double-click action harmful software. By disabling the default fade the known endings falls on such a tried obfuscation. The Displays and hides the extensions can also affect the behavior of VBA scripts.

In the WWW, where files are transferred via the Hypertext Transfer Protocol, is not the file extension, but being broadcast MIME type concerned, but which is, in turn, usually determined from the extension. In embedded images, most browsers determine the type based on the magic number.

On IBM mainframes, the file extension is used (also called low level qualifier here ) only to assign the correct SMS constructs in the allocation (Data class, management class, storage class ). In addition, the ISPF editor saves its profiles per file extension. The data format itself is stored in the VTOC or in the VSAM catalog or in the tape header.