File extension

In computing, the term file extension (also, file extension) is a string appended to a file name, usually predicted by a period. Its main function is to distinguish the content of the file, so that the operating system has the necessary procedure to execute or interpret it. However, the extension is only part of the file name and does not represent any kind of obligation with respect to its content.

Some operating systems, especially DOS heirs like Windows, use file extensions to recognize their format, even executable files. Other operating systems, such as those based on Unix, use the file extensions by simple convention, and do not necessarily use them to determine the file type.

Since the legacy file extensions are from the DOS system, many of their current features were inherited due to limitations in that system. Older DOS systems limited the number of characters in a file extension to three, so many conventional extensions have that many characters. Also, file names on DOS systems are case insensitive, so most file extensions can be written in lowercase or uppercase or a combination of both.

Due to its extension, the same basic name can contain files of different purposes. As in this DOS example:

APPLICATION.BAS Source file written in BASIC language.

APPLICATION.OBJ Object file (required for your compilation).

APPLICATION.EXE Executable Program.

FILE.XXX ASCII text file "plain text".

FILE.RTF A formatted text file.

FILE.DOC A text file in Microsoft Word format.

FILE.VBS Source file written in Visual Basic language.

In all of these cases, the extensions both differentiate file names and identify them to applications that can handle them, making it easier to get results.

History

On Unix systems, file names can contain any character except the directory separator ("/") and usually have no or acceptable limit on their size. The extension is not a distinct element but a convention given to file names to recognize their content.

In DOS, the names of the files had a maximum of 8 characters, a period and an extension of a maximum of three letters, in total a maximum of 12 characters of the ASCII Code that allowed distinguishing some files from others within the same directory. Not all ASCII code characters were allowed to name a file, as some (example: /*?+ and others) were reserved for other functions. There is no difference between upper and lower case.

In Windows 95, the limits imposed by DOS on the number of characters in the file name and extension were removed.

There are no extensions in Apple's Mac OS Classic. Finder associates files with a four-character Filetype code to each file with a specific program or action, for example the code TEXT means that it is a text file and can be opened with any text editing software. As of OS X the Uniform Type Identifier is used which handles extensions identically to Microsoft Windows. The Filetype was deprecated with OS X 10.6 Snow Leopard.

In Commonores files can only have four extensions: PRG, SEQ, USR, REL. However these are to separate data types used by a program and are irrelevant to identifying program contents.

Types of extensions

System plugins

These are the file extensions necessary for the internal functioning of the Microsoft Windows operating system as well as the different programs that work on it. It is not recommended to move, edit or change them in any way as it could affect the operation of the system. They are named in alphabetical order.

Video Extensions

Video files are typically played in movies or on web pages that contain videos. The most common are:

Audio Plugins

Audio files are those that play sound when executed. The most common are:

Image Extensions