Usage
Filename extensions can be considered a type of metadata. They are commonly used to imply information about the way data might be stored in the file. The exact definition, giving the criteria for deciding what part of the file name is its extension, belongs to the rules of the specific filesystem used; usually the extension is the substring which follows the last occurrence, if any, of the dot character (example: txt is the extension of the filename readme.txt, html the extension of mysite.index.html). On file systems of mainframe systems such as MVS, VMS, and PC systems such as CP/M and derivative systems such as MS-DOS, the extension is a separate namespace from the filename. Under Microsoft's DOS and Windows, extensions such as EXE, COM or BAT indicate that a file is a program executable.
The UNIX-like filesystems use a different model without the segregated extension metadata. The dot character is just another character in the main filename, and filenames can have multiple extensions, usually representing nested transformations, such as files.tar.gz. This model generally requires the full filename to be provided in commands, where the metadata approach often allows the extension to be omitted.
With the advent of graphical user interfaces, the issue of file management and interface behavior arose. Microsoft Windows allowed multiple applications to be associated with a given extension, and different actions were available for selecting the required application, such as a context menu offering a choice between viewing, editing or printing the file.
Pre-OS X versions of the Mac OS disposed of filename-based extension metadata entirely, instead using a distinct file type code to identify the file format. Additionally, a creator code was specified to determine which application would be launched when the file's icon was double-clicked. Mac OS X, however, uses filename suffixes, as well as type and creator codes, as a consequence of being derived from the UNIX-like NeXTSTEP operating system.
Read more about this topic: Filename Extension
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