In amongst the barrage of jargon and mumbo-jumbo facing newcomers to the joys of personal computing is a tricky little beggar called the 'file extension'. This is the three or four letter/number code that comes after the stop or dot in a file name. So what are they for and what on earth do they all mean?

The first bit is easy, a file extension tells the operating systems – Windows in other words -- which program the file belongs to, or is 'associated' with, and it can instruct Windows to perform a particular action. For example, if you open a folder in Windows Explorer and double-click on a file icon with the extension '.doc', Windows recognises that the file contains a formatted text document created using Microsoft Word so it automatically opens Word and displays the document on the screen.

The meaning of an extension like .doc is fairly obvious but how about the others? There are more than a thousands different file extensions and it would be pointless, not to say nigh-on impossible to learn the meaning of them all, in any case you're unlikely ever to see more than a small fraction of them in normal use. For your information we've included some of the commonest ones in the Jargon Filter box below. Windows does its best to keep most types of files hidden from your view. In fact you rarely need to bother about file extensions at all, unless you use fairly exotic applications, make a habit of poking around in Windows Explorer or something goes wrong.

That can happen when Windows encounters a file extension it doesn't recognise and it displays a dialogue box asking you to associate it with a program. A lot of error messages refer to a file that's missing or corrupt. If that happens there's an easy way to find out what a file extension means and what program it belongs to – assuming you haven't removed it -- using a utility in Windows Explorer. Open Explorer, click on the View menu and select Folder Options. Choose the File Type tab and you'll see a list of all of the files Windows recognises and the programs they're associated with. It's worth spending a few minutes looking through the list, it makes interesting reading and tells you a lot about the programs you have installed on your PC.

One thing the File Type dialogue box doesn't tell you is what's in a file. Broadly speaking a file will normally contain one of two types of data, binary information – lots of ones and zeros – or ASCII (American Standard Code for Information Interchange) characters – letters and numbers. If you want to peek inside a file you will need a suitable viewer, Windows comes with one, albeit with limited capabilities, called Quick View and it can display around 40 different file types. To use it right-click on a file icon in Windows Explorer, if the file can be viewed Quick View will appear on the drop down menu; click on it and the contents will be displayed. If Quick View isn't shown it may not be installed, if so go to Add/Remove Programs in Control Panel, click on the Windows Set-up tab, double click Accessories, check the Quick View box and follow the instructions.    

One of the most frequent problems with extensions is when file associations change and programs start behaving differently. Some types of files can be read or processed by several different programs, for example image files with the \extension '.bmp' or '.jpg' can be viewed by most paint and graphics programs and later versions of Windows Paint. However, if you install a new graphics program it might take over all image file associations and whereas in the past if you clicked on a .jpg file it opened and displayed on PaintBox Pro, you will find that in future it opens the new program. You easily can change the association back to what it was by going to the File Type dialogue box. Scroll down the list until you come to the particular file type, click to highlight the entry and select Edit. On the Edit File Type window that opens click on the Edit button and use the Browse facility to change the program or action it is associated with.

Finally a word of warning, Windows Explorer will allow you change most file names and extensions simply by clicking into the name field next to the icon or listing. Don’t, unless you know exactly what you are doing! Changing file extensions can be very dangerous and lead to all sorts of problems. If you must fiddle around with file names always make a copy and give the original the extension '.old'.

Next week – Internet Domain names




Audio-Video Interleaved, Microsoft standard movie files


Backup or archive file, usually created automatically by a program


Bitmap, standard Windows image or graphics file


Cabinet, compressed data file used on Microsoft software installation discs


Dynamic Link Library, contains information or data that may be shared by several programs


Microsoft Word document


Executable, a file containing a program or instructions to start a program


Graphics Interchange Format, a graphics file, mainly used on Internet web pages


Help file


HyperText Markup Language, Internet web page files


Windows icon files


Initialisation file containing information needed to start and configure Windows


Joint Photographic Experts Group, compressed image file  


Lotus Word Pro document


Musical Instrument Digital interface, music file


QuickTime Movie file


Moving Picture Expert Group 3, CD quality sound file used for music on the Internet


Moving Picture Experts Group, video movie file


Convention for renaming old or disused files that may be needed at some time in the future


Portable Document Format, interactive text file with web-like links


Rich Text Format, industry standard text file, can be read by most word processors


Screensaver file


System file, containing information needed to load and configure Windows


Tagged Image Format File, graphics file


Temporary file, generated by Windows and various applications, normally deleted when the program or Windows is closed


True Type Font, file containing typeface information



File containing plain or unformatted text


Text file format used to send program files containing binary information by email


Waveform, windows sound file


Word Perfect document file


Microsoft Excel worksheet


File containing compressed binary data, used for sending programs or information on the Internet



Heavy-duty Internet users, here's a way to save yourself several seconds a week by increasing the dialling speed of your PC and modem. It may not work with some modems or phone lines but it's worth a try. Go to Control Panel click on the Modem icon, then Properties and select the Connection tab and click on Advanced. In the Extra Settings field enter S11=50 then click OK. S11determines the duration of each tone pulse, in milliseconds, the second number specifies the gap between each tone, thus reducing the number to 45 say, makes it dial even faster, increasing the number slows it down. If the connection fails or becomes unreliable simply clear the Extra Settings field to return to the default values.

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