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 .

jar: A Java archive should be usable on any machine with Java.
java: A file with this extension should be a file written in the Java programming language. It should be in simple ASCII (although also possibly Unicode) and should be usable on any machine with a Java compiler.
jbig & jbg: An image format developed by the Joint Bi-level Image Experts Group. It is optimized for FAX type images, and has absolutely nothing to do with either JPEG or MPEG in spite of the similarity of names. It is lossless and free viewing programs are available for it for most platforms, but it is not directly recognized by most browsers.
jhtml: Very similar to shtml but utilizes Java to do its work.
jpeg & jpg: An image format optimized for "natural" images developed by the Joint Photographic Experts Group, JPEGs are probably second only to GIFs in level of acceptance. JPEGs manage to capture wonderfully detailed images in millions of colors in minimal space by taking advantage of limitations with human vision, and performing little lossy compressions. This means that each time a JPEG is saved, it will lose a little more quality, although each individual loss will be nearly invisible to the human eye. If care is not taken, however, a JPEG image can become worse looking than a color-limited GIF. JPEGs are also not good at storing cartoon-like images or line drawings; for these cases either GIFs or PNGs or X-bitmaps are better choices. JPEGs also cannot contain a transparent color. If a transparent color is needed, GIFs, PNGs, and X-bitmaps are better choices. JPEG has nothing to do with either JBIG or MPEG in spite of the similarity of names.
js: A file with this extension is probably written in the JavaScript programming language for browsers. It should be in simple ASCII and should be usable on any machine with a browser that supports JavaScript. Note that there is no relationship between JavaScript and Java; it is an unfortunate coincidence of names.