Saugus.net: Glossary of Computer File Extensions

Filename Extensions List

So you see a file somewhere on the 'net with a name like "greatfile.xyz" and you would like to download it and use it on your computer. Will it work? What does the file extension xyz mean, anyway? This handy guide attempts to provide answers to those questions.

It is not meant to be read straight through; rather, it is meant to be a reference. An extension may be looked up either with the "find in page" option of your browser or by appending a "#xyz" (without the quotes and with xyz replaced by the extension in question) to the "go to" or "URL" field on your browser. There is also a Search Interface that will return not only the specific extension sought but also other entries that reference it. Be aware though that it assumes familiarity with the computer basics discussed on the terms page.

You may notice that most extensions are three letters (or fewer) long. This is due to a historical limitation of the operating system called CP/M (that was later inherited by MS-DOS). In fact, the whole concept of file extensions comes from CP/M. Most modern operating systems do not attribute any special meaning to the "." (period, or dot) character.

Be aware though that there is no standardization to filename extension usage, and many different people have used extensions to apply to many different things. This list only attempts to provide likely guesses of what something is apt to be. Programs that can make use of many of these extensions can be found on the Guide to Free Software.

If you want something added or see a problem with something already here (but keep in mind this guide is not meant to be overly technical) please send .

x3d: An Extensible 3-Dimensional Graphics file contains XML data representing a 3D image. It can be viewed by any application that can understand X3D data.
xbm: The X bitmap format was designed specifically for small icons. It is in simple ASCII and only supports monochrome. It is supported on many platforms and most browsers. It is not space efficient, but because it is always used for small images, this is usually not a problem. Historically it is the original World-Wide Web image format; the others came along later.
xcf: The Experimental Computing Facility format is used by the GIMP to store image files.
xls: An Excel spreadsheet document; can be read on any machine with Microsoft Excel (available only for WinTel boxes and Macs, but usually not for free). Note that in newer versions of Excel Microsoft has split this format into two variants that use either the XLSX or the XLSM extension.
xlsm: This extension is used by one of Microsoft's replacements for the XLS format. It can only be read by machines with MS-Excel (at least version 12) installed. Note that unlike the XLSX variant, these files can contain embedded code (the "m" stands for "macro") and should thus not be considered safe if they come from untrusted sources.
xlsx: This extension is used by one of Microsoft's replacements for the XLS format. It can only be read by machines with MS-Excel (at least version 12) installed. It is a safer sibling to the XLSM format.
xml: An extensible markup language file contains data in simple ASCII that can be read with particular applications on a case-by-case basis. Much of the data contained within the World-Wide-Web is actually in this format.
xo: An "activity" bundle for a OLPC project XO laptop. It is typically an application often (but not always) written in Python bundled into in a single zip file along with all its supporting data.
xpi: An extension package installer used by Mozilla and several related browsers. It will generally perform the install automatically if entered as a URI.
xpm: The X pixmap format was designed specifically for small icons. It is in simple ASCII but supports multiple colors, even a transparent color. It is supported on many platforms and most browsers. It is not space efficient, but because it is always used for small images, this is usually not a problem.
xps: XPS stands for XML Paper Specification and is quite similar in intent to PDF. Being newer, it does not yet have the widespread support that PDF enjoys, though.
xul: An XML user-interface language file contains XML data that can be parsed by applications that understand XUL.
xwd: An X-Windows dump is the X-Windows variant of the bmp and pict image file formats with similar limitations.