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{{more citations needed|date=November 2015}}
A '''filename extension''', '''file name extension''' or '''file extension''' is a suffix to the [[filename|name]] of a [[computer file]] (for example, <code>.txt</code>, <code>.docx</code>, <code>.md</code>). The extension indicates a characteristic of the file contents or its intended use. A filename extension is typically delimited from the rest of the filename with a [[full stop]] (period), but in some systems<ref>{{cite manual
| title = z/VM
| id = SC24-6265-
| section = What Is a File?
| section-url = https://www.ibm.com/servers/resourcelink/svc0302a.nsf/pages/
| page = 7
| quote = One thing you need to know about creating files with z/VM is that each file needs its own three-part identifier. The first part of the identifier is the file name. The second part is the file type. And the third part is the file mode. These three file identifiers are often abbreviated fn ft fm.
| date =
| url = https://www.ibm.com/servers/resourcelink/svc0302a.nsf/pages/
| publisher = IBM
}}
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== 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>
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.
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