(About 610 words)

The Mystery of File Extensions Explained
By Dave Gerber, Advisor, Region 1, Dave’s Bits & Bytes, A Periodic Newsletter for the Members of the Sarasota PC User Group and the World!!   
Director, Sarasota PCUG, Florida
www.spcug.org / http://davebytes.com/

GIF, JPG, TXT, DOC, HTML, WAV, BMP, ETC

You'll see them over and over again as you encounter manuals, web sites, and anything related to your computer. Tons of file extensions. Whole pickup truck loads of 'em running back and forth on the info highway.

File extensions are easy enough to understand, and with just a bit of history, you'll know everything you need to know about files, and not a bit more.

Back in the DOS days (DOS stands for Disk Operating System), before the invention of Windows, every file had to be named with a maximum of 8
characters, and could include a three letter "file extension."

For example, lets say you want to save that recipe for Quevos Rancheros. You were forced to use a maximum of 8 characters. So you call it queranch.
Hmm, in Spanish, that's like, "What ranch?" Or that's a loose translation, at least.

You were limited. Severely. So along comes the long file name. You can use a whole mess of characters now, calling your file "My greatest recipe
using eggs and tortillas since the invention of the wheel." The sentence between the quote marks is the name of the file, or "filename."

The stated character limit is 255, but in reality it comes in just a tad under, like 253 or something. The techies can quibble over that. Regardless,
it's l-o-n-g. And more than adequate to describe the content of your creation!

Now, notice that many files have extensions. They all do, really. In fact, your computer doesn't know what to do with a file if it doesn't have an extension.

The extension is the three letter part following the main name. (You didn't see an extension on my quevos rancheros example, because I left it off.)
Here's an example:

My Word processing report.doc

The ".doc" part of the file's name tells Windows to use the program that's associated with .doc files to open it. So, let's say you've got Microsoft
Word installed. Whenever the Windows operating system realizes you've clicked a file with the extension ".doc" it fires up Word, and Word opens the
file.

Presto.

There are lots of file extensions. Here's a table for you that includes a few common file types you may encounter during your web travels.

 

Common file types

Extension Type of file 
txt Text File, such as Notepad produces 
doc Document File, for example, MS Word 
gif Graphics Interchange Format (graphics) 
jpg Joint Photographic Experts Group (graphics) 
pdf Portable Document File 
htm Hypertext Markup (special coded text files) 
html Hypertext Markup Language (ditto) 
zip Compressed file requiring special software to decompress the file 
tar Same as zip. Just another compression style 
pdf Portable Document File (universal text file) 
wav Sound files 
bmp Bitmap files (graphics) 

 If you try to open a file extension that's unregistered (unregistered means Windows doesn't know what to do with it 'cause there's no association in the Windows Registry), Windows throws the Open With dialog box at you

This dialog box lets you decide what program to open the file with. For example, I tried to open the file named 'java.usj' and I got this screen as a
result. Since nothing on my computer is set up to edit or open a '.usj' file, I have to locate and use a program that "understands" that type of
file extension. Now that's another trick altogether!

Commonly, people will send Power Point presentation files or Microsoft Publisher files as attachments to people who don't have Microsoft Office
installed on their computer. So those files cannot be viewed unless the appropriate software is installed.
 
There is no restriction against any non-profit group using this article as long as it is kept in context with proper credit given the author.  The Editorial Committee of the Association of Personal Computer User Groups (APCUG), an international organization of which this group is a member, brings this article to you.