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 .

m: A source file written in the Objective-C programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with an Objective-C compiler. There is a good chance though that it will also require (at minimum) some h files.
M & mm: A source file written in the Objective-C++ programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with an Objective-C++ compiler. There is a good chance though that it will also require (at minimum) some H or (h) files.
m4b: This is most likely an MPEG-4 container specialized for audio books. It's basically the same as an mp4 file but with additional features (such as bookmarks) convenient for audio books.
mdb: A Microsoft database file is a binary file containing a database for use with Microsoft Access. It is not portable and in fact it requires specialty tools to extract the data.
mf: The MetaFont format is used to store font information. It will work on any machine that has TEX installed.
midi & mid: A musical instrument digital interface file is an audio file that will possibly work with any device that is MIDI aware. MIDI is in some respects similar to MOD, but it is supported on far more dedicated musical devices but far fewer computers.
mif: A Maker interchange format file is a Framemaker file with an advertised format so that it can be imported into other desktop publishing or word processing programs. The ability to handle MIFs is built into better word processors and desktop publishers. It will behave the same across different platforms.
miff: An image format used with both the ImageMagick and GraphicsMagick suites of image processing programs. It can be used on any platform that supports either ImageMagick or GraphicsMagick.
mng: A multiple-image network graphics file is used for storing animations. It is binary and related to the png format.
mod: The audio module file originated on the Amiga and takes the approach of defining different instruments and then describing what each does. It is fairly space-efficient, and free software to play MODs is available for WinTel boxes, Macintoshes, UNIX boxes, and even Commodore 128s in addition to the original Amiga.
mov & qt: A QuickTime movie can actually be used to store both movies and sounds. A file with this extension could thus be either an audio file or a movie file. Both types are supported by many different platforms. Note though that there are several different versions of QuickTime files in use, and not all players will play all versions.
mpeg, mpg, mpeg2, mpeg3, mpeg4, mp2, mp3, mp4, m3u, & m4u: The Moving Pictures Expert Group devised formats for storing both movies and sounds. A file with this extension could thus be either an audio file or a movie file. Both types are supported by many different platforms, with perhaps the audio type being a little more recognized. Both types also utilize lossy compression that is designed to take advantage of the limitations of human vision and hearing. Regardless the compression still occasionally does weird things producing "MPEG compression funnies", particularly during periods of rapid movement in video. MPEGs are not limited to computers; some digital satellite television transmissions use an MPEG format, and DVDs use an MPEG format. MPEG has nothing to do with either JPEG or JBIG in spite of the similarity of names. MPEG does have some relation to QuickTime, though, and the next version of each will work to merge the two. If there is a number at the end of the extension, it refers to the specific generation of the MPEG format in use within the file.