Editing Filename extension (section)

== Content type ==
Filename extensions may be considered a type of [[metadata]].<ref name="Stauffer et al 2017">{{cite book|last1=Stauffer|first1=Todd|last2=McElhearn|first2=Kirk|title=Mastering Mac OS X|date=2006|publisher=John Wiley & Sons|isbn=9780782151282|pages=95–96|url=https://books.google.com/books?id=62xkJo6JXwAC&pg=PA95|access-date=2 October 2017|language=en}}</ref> 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 [[file system]] used; usually the extension is the substring which follows the last occurrence, if any, of the [[full stop|dot character]] (''example:'' <code>txt</code> is the extension of the filename <code>readme.txt</code>, and <code>html</code> the extension of <code>index.html</code>).
On file systems of some mainframe systems such as [[Conversational Monitor System|CMS]] in [[VM (operating system)|VM]], [[OpenVMS|VMS]], and of 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 [[Microsoft Windows|Windows]], extensions such as <code>[[EXE]]</code>, <code>COM</code> or <code>BAT</code> indicate that a file is a program [[executable]]. In [[OS/360 and successors]], the part of the dataset name following the last period, called the low level qualifier, is treated as an extension by some software, e.g., [[Time Sharing Option|TSO]] EDIT, but it has no special significance to the operating system itself; the same applies to Unix files in MVS.

The filename extension was originally used to determine the file's generic type.{{citation needed|date=November 2015}} The need to condense a file's type into [[8.3 filename|three characters]] frequently led to abbreviated extensions. Examples include using <code>.GFX</code> for graphics files, <code>.TXT</code> for [[plain text]], and <code>.MUS</code> for music. However, because many different software programs have been made that all handle these data types (and others) in a variety of ways, filename extensions started to become closely associated with certain products—even specific product versions. For example, early [[WordStar]] files used <code>.WS</code> or <code>.WS''n''</code>, where ''n'' was the program's version number. Also, conflicting uses of some filename extensions developed. One example is <code>.rpm</code>, used for both [[RPM Package Manager]] packages and [[RealPlayer]] Media files;.<ref>[http://filext.com/file-extension/rpm File Extension .RPM Details] from filext.com</ref> Others are <code>.qif</code>, shared by [[DESQview]] fonts, [[Quicken]] financial ledgers, and [[QuickTime]] pictures;<ref>[http://filext.com/file-extension/qif File Extension .QIF Details] from filext.com</ref> <code>.gba</code>, shared by [[GrabIt]] scripts and [[Game Boy Advance]] ROM images;<ref>[http://filext.com/file-extension/GBA File Extension .GBA Details] from filext.com</ref> <code>.sb</code>, used for [[SmallBasic]] and [[Scratch (programming language)|Scratch]]; and <code>.dts</code>, being used for [[Torque Game Engine|Dynamix Three Space]] and [[DTS (sound system)|DTS]].

=== Compared to MIME type ===
In many [[Internet protocol suite|Internet]] protocols, such as [[HTTP protocol|HTTP]] and [[MIME|MIME email]], the type of a bitstream is stated as the [[media type]], or MIME type, of the stream, rather than a filename extension. This is given in a line of text preceding the stream, such as ''Content-type: text/plain''.

There is no standard mapping between filename extensions and media types, resulting in possible mismatches in interpretation between authors, web servers, and client software when transferring files over the Internet. For instance, a content author may specify the extension ''svgz'' for a compressed [[Scalable Vector Graphics]] file, but a web server that does not recognize this extension may not send the proper content type ''application/svg+xml'' and its required compression header, leaving web browsers unable to correctly interpret and display the image.

[[BeOS]], whose [[Be File System|BFS]] file system supports extended attributes, would tag a file with its media type as an extended attribute. Some [[desktop environments]], such as [[KDE Plasma]] and [[GNOME]], associate a media type with a file by examining both the filename suffix and the contents of the file, in the fashion of the [[file (command)|file]] command, as a [[heuristic]]. They choose the application to launch when a file is opened based on that media type, reducing the dependency on filename extensions. [[macOS]] uses both filename extensions and media types, as well as [[OSType|file type codes]], to select a [[Uniform Type Identifier]] by which to identify the file type internally.