TUCOWS ARTICLE

Understanding File Extensions

In order to tell you about file extensions, and how to understand what they mean, it's necessary to give you a little bit of the history of file extensions. What is a file extension? Well, it's those extra characters, normally 3 of them, that come at the very end of a file name, after the period (.) --- so, in the file name TESTING.DOC, the .DOC would be the file extension. I say "normally 3 of them" because is some rare cases there may be only 1 or 2. Now, the next question is "What are they for?" and that's a good question.
Published: Jun 6, 2005
Author: Michael E, Callahan

In order to tell you about file extensions, and how to understand what they mean, it's necessary to give you a little bit of the history of file extensions. What is a file extension? Well, it's those extra characters, normally 3 of them, that come at the very end of a file name, after the period (.) --- so, in the file name TESTING.DOC, the .DOC would be the file extension. I say "normally 3 of them" because is some rare cases there may be only 1 or 2. Now, the next question is "What are they for?" and that's a good question. In The Beginning...

In the early days of computers file extensions were used so users, and programmers, would know what certain files did. What kind of data they contained. So, you could know that files with certain extensions would contain certain formats of information. For example, a TXT file would be a plain text file, whereas a BIN file would be a binary file that would only be used by a program or the operating system. Back in the early 1980's the most common file extensions were EXE (executable), COM (command), BAT (batch), BIN (binary), and a few others.

Some files, like those that were part of DOS, had specific, set file extensions. At the same time, however, the file extensions used by other programs went off in all kinds of directions. Most programs began to create their own file extensions, primarily so the user could easily tell which program had created the file. So, for example, a WordStar document had an extension of .WS (for WordStar), an early spreadsheet had a file extension of .WS1 (for worksheet) and so on. One of the big problems, with file extensions and file names, was the strict limitations that were in place under DOS and the early versions of Windows.

Advent Of Long File Names

For all of you who came into computers since the release of Windows, you're most likely used to having the luxury of long file names. But back in the "old days" there was only what was affectionately known as the "8 and 3" file names. The famed "8 and 3 convention." Your file name could only have eight characters in it, and then you could have three characters in the file extension. And this was still true with the first versions of Windows. Having this convention in place led to some pretty innovative names for files, especially if you wanted to know one from another. Where today you could name a file "Disk Contents of Drive C", back then you'd have had to be happy with "DSKCNTSC.TXT". I have to be honest and say that it took me a long time to get used to being able to use long file names. And, in fact, many of my file names still use the 8 and 3 convention. Old habits die hard. Long file names give you the luxury of giving really informative names to your documents and data files like "Tax Report 2004.XLS, Summary of Meeting of Board of Directors.DOC and the like. At the same time, the number of file extensions is just about overwhelming.

Extension Expansion

As more and more software was created, the number of file extensions grew and grew. Early programs that dealt with file compression had extensions like .LBR, .LQL, .ARC, and .ZIP. Graphics files started out with things like GIF and PCX and expanded to encompass extensions like JPG, PNG, BMP, RAS, TIF, PGM, FSH, ICO, PCM, JPM, LDF, PBM, PGA, RAW and many, many more. File extensions have continued to be proprietary in nature. A .DOC file was from Microsoft Word, while an XLS file was from Excel. Today, there are so many file extensions that it's almost impossible to keep up with all of them. Most people come to know the file extensions they use most -- the ones created by the programs they use. The problem arises, however, when you get a file from someone else and you have no idea what it is or what program will open it. The growth of the Internet has simply added to the number of file extensions. You have things like HTM, HTML, SHTML, XML, ASP, PHP, CGI, JSP, SGML, VBS, and more. And, if file extensions weren't puzzling enough in and of themselves, they are now being used for another purpose.

File Extension Caution

Recently, the clever people who create worms and viruses have taking to altering file extensions as a way of tricking the public. And, to make matters worse, Microsoft Windows lets you turn off the ability to view file extensions. So, some "bad guys" have taken advantage of this by giving their files two file extensions. Like this:VACATION.TXT.EXESUMMER.TXT.VBSLooks innocent enough, huh? BUT, if you have extensions turned off these same files would show up to you as SUMMER.TXT and VACATION.TXT. If you go to open them they actually run programs because they aren't text files at all. It's actually a way to spoof a file extension just as some Web domains are spoofed by those trying to get your personal information. The wisest thing to do is to make sure you have Windows set up so you can view all file extensions. This is very easy-to-do and there are actually a couple of ways you can do it. Under Windows XP, just click Start... Settings... Control Panel... and Folder Options. Then under Folder Options just click the "View" tab. Look down and you'll see a number of options, but look for the one that says "Hide file extensions for known file types" and make sure it is NOT checked. Then click "OK" and you're done.

What's This?

Over time, all of us, including me, are going to encounter file extensions and not know what they are. My rule for this is "When in doubt, LOOK IT UP! That's what I do -- there's certainly no shame in looking up a file extension you don't know any more than there is in looking up a word you don't know. There are several sites where you can go to find relatively up-to-date listings of file extensions. These sites normally list the extensions and tell you what program created it. A few sites you can use to look up file extensions include:

You can visit any of these sites and find out just what program a file extension comes from. What created it? Is it a known file extension? And so on. Use more than one site if you need to. Make it a point to know the file extensions created by the programs you use. Make sure that you can see the file extensions. And never be afraid to look one up if you don't know it.


About Michael E, Callahan

Michael E. Callahan, known around the world by the trademarked name Dr. File Finder, is regarded as the world's leading expert on shareware. Dr. File Finder works with software programs and developers full-time, and in the average year he evaluates 10,000 programs. Since 1982 he has evaluated over 250,000 software and hardware products. Mr. Callahan began evaluating software online in 1982 and no one has been at it longer. He currently works doing online PR and marketing for software companies, and is the Senior Content Producer for Butterscotch.Com.

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