File Types  

What type of files are on your hard drive? 

Every day you create and delete files on your computer, install and remove 
programs that add and remove files, copy files, rename files, etc. You have 
tens of thousands of files on your hard drive. After a basic clean 
installation of Windows 98, you'll already have well over 2000 files. To 
see how many files you have right now, open up Windows Explorer by going to 
Start/Programs/Windows Explorer. Click once on your hard drive in the left 
column. Now hit Ctrl-A on your keyboard, which will highlight all files and 
folders on the right. Then right-click on the highlighted files and select 
Properties from the menu. In the resulting window, you'll see at the top 
the number of files. More than you thought, isn't it? 

Now if you take a look at the list of files on the right, you'll see rows 
of folder icons, followed by rows of file icons. Each file has its own 
unique name, since you cannot have two files with the same name in the same 
folder. If you're looking at the root directory of your C: drive, you'll 
probably see a few file names such as Autoexec, Command, etc. Some others 
might have pretty cryptic names such as RG2CATDB or SFCSYNC. But what does 
that mean and what do those files do? Read on if you're curious what files 
there are, what they do and how to identify them easily. 

Hide-and-seek 

When you install Windows and then open Windows Explorer, you won't see 
much. By default, Windows hides a lot of information about your files as 
well as dozens of files from you to reduce the risk of your operating 
system being compromised by your messing around with them. But since we're 
the curious type, let's change a few settings so we can start looking 
behind the scenes. 

After installing Windows, the very first thing I do is open Windows 
Explorer, go to View/(Folder) Options (Tools/Folder Options in Windows 
2000) and select the View tab. Here you'll see a list of radio buttons and 
check boxes. The first one I modify is labeled Hide file extensions for 
known file types. Uncheck it. The next one we're interested in is the 
section labeled Hidden files. Select the radio button Show all files. 
That's all we're doing here for now, so click OK. When you're back to the 
Explorer window, go to the View menu and select Details. Now look at the 
list of files and folders on the right. It should look a lot different now. 

A different view 

Suddenly you see files you didn't see before. Those new files are system 
files. Windows by default hides system files from you to avoid them being 
deleted by accident because that could seriously mess up the operating 
system up to the point where you cannot boot anymore and have to reinstall 
Windows. 

Another thing you should notice is that almost every file now has a dot 
after the file name followed by three more letters. That is called a file 
extension. These extensions make it a lot easier to identify a file, see 
what type of file it is, what it does, and what program it is associated 
with. The following table shows a list of the most common file extensions 
and explains what they mean: 

Extension What it means 
avi Video clip - A type of video file format 
bak Backup - When a program makes changes to an important file, it should 
make a backup first. Bak is a common extension to indicate a backup file 
bmp Bitmap - A type of graphics file containing an image 
cab Cabinet - An archive file containing compressed files, usually used by 
installation programs to store compressed setup files 
dll Dynamic Link Library - A critical file to Windows and applications. It 
stores additional commands and information for applications that are called 
from the program as needed. If a dll file is deleted, the program probably 
will not work anymore and needs to be reinstalled 
doc Document - A Microsoft Word document 
exe Executable - This file is an actual application that can be launched. 
For example, Notepad.exe contains the Windows Notepad application 
gif Graphics Interchange Format - A type of graphics file containing an 
image 
hlp Help - This is where Windows and applications store the information 
that you get when you access the Help menu 
html Hyper Text Markup Language - one of the several languages used to 
create documents for the Internet 
ico Icon - The little pictures next to filenames or on your desktop are 
sometimes stored in this type of file 
inf Setup Information - This file type comes with most applications and 
drivers. It contains information that Windows needs to install that 
application or driver 
ini Initialization file - Windows as well as most applications store some 
configuration settings, e.g. user preferences, in ini files. If such a file 
is deleted, the program probably will still work, but all custom settings 
are lost 
jpg/jpeg Joint Photographic Experts Group - A type of graphics file 
containing an image 
log Log - Some applications keep track of certain activity, it is stored in 
a log file 
mid/midi Musical Instrument Digital Interface - A music industry standard 
for electronic forms of music 
mp3 Moving Picture Experts Group Audio Layer 3 - Compressed audio file 
mpg/mpeg Moving Picture Experts Group - A type of digital compression 
standard 
pdf Portable Document Format - A cross-platform file format developed by 
Adobe 
tif Tagged Image Format - A type of graphics file containing an image 
tmp Temporary file - A lot of times Windows or other applications will save 
some information temporarily on the hard drive. Normally those files are 
supposed to be deleted when they're no longer needed, but sometimes they 
remain. Usually these files can be removed 
txt Text - This indicates a plain ASCII text file that can be read by 
pretty much every word processing application 
wav Wave - Another type of sound file 
zip Zip - An archive file storing compressed files 

If you want to know about other file extensions you didn't see listed here, 
check out http://www.webopedia.com where you can search for file extensions 
and many other computer related terms. 

Now that you have an idea what file extensions are and can identify the 
most popular ones, let's take a closer look at what those extensions are 
good for. 

The meaning of extensions 

Extensions help Windows understand what to do with a file. Extensions tell 
Windows what program to use to open a file or what action to perform when a 
file is double-clicked. Some files, e.g. exe files, are full-fledged 
programs that don't need any help from Windows to run (in a simplified 
manner of speaking). Other files cannot do anything by themselves, they 
need an application that can open them and make them useful. To open 
graphics files, you need software that can open that type of image file. To 
open text files, you need software that is capable of word processing. 

Let's take text files for example. Go to Start/Find/Files or Folder, type 
*.txt, select Local hard drives and click Find Now. You should get a long 
list of text files that are on your hard drive. Pick a text file from the 
list and double-click on it. Most likely an application called Notepad will 
pop up on your screen and display the text that is contained in the text 
file you selected. But how did Windows know how to do this? 

Register it 

The clue is, as with everything that Windows does, in the Windows registry, 
the heart and soul of Windows. This is the place where Windows stores a 
list of file types it knows, what these files do, and what to do with them 
when one is selected. Thankfully we don't have to hack around in the 
registry to take a closer look at this list and to modify it. There is a 
relatively simple interface for that. Open Windows Explorer, go to View/
(Folder) Options, and select the File Types tab. Here you'll see a long 
list of registered file types that you can scroll through. Take a moment 
and browse through this list. You'll probably see all the file types I 
mentioned earlier and a whole lot more. 

For our example, let's scroll down to the T's and find the listing for txt. 
Click on it once and observe the details displayed below. You'll see that 
Windows indicates that the extension for this type of file is txt and it 
opens with Notepad. Now we know why Windows automatically opened Notepad. 
When you double-clicked the file earlier, Windows looked at the extension, 
compared it against its list of registered file types, found it, and 
followed the instructions and opened it with Notepad. 

How did this list come into existence? Windows knows right out of the box 
how to recognize a bunch of different file types. In addition, every time 
you install a new piece of software that uses a certain type of file, 
during the installation of this program the required entries are made into 
this list. This process is called registering a file type. 

More options 

But there's more. There are more options than just to open a file. If you 
right-click on a text file in Explorer, you'll notice that the top two 
items on the context menu are Open and Print. OPen is highlighted because 
it is the default action to perform for this file type. Print is an 
additional option and easily accessible via this convenient shortcut in the 
context menu. Instead of having to open the file with its associated 
application and going to File/Print, just right-click and select Print. 
Very easy. 

Now your next thought is probably "Cool! And how can I add my own custom 
options to this right-click menu, or change the existing option?" You'll be 
surprised how easy this is. Open Windows Explorer, go to View/(Folder) 
Options, and select the File Types tab. Scroll down to txt, highlight it 
and click Edit, or just double-click it. Up pops a windows that lets you 
edit this file type. If you look in the Actions field, you'll notice two 
options listed: Open, which is bold indicating it is the default option, 
and Print as a secondary action. In this dialog you can also change the 
default option in the right-click menu by highlighting an action other than 
the current default action and pressing Set Default. 

Let's say you wanted to have another option of easily editing a text 
document with Word. Click New and you'll get a dialog box where you can 
create your own action. First type in a name for the new action, but try to 
keep it short, otherwise the context menu gets too wide. Type Edit with 
Word. The field below is where you specify the path to the application that 
will be used for this action. Click the Browse button and navigate to C:
\Program Files\Microsoft Office\Office\WinWord.Exe (or wherever Word is 
installed on your PC) and double-click WinWord.Exe. That will put the path 
into the application field. That's it, you're done. Just keep clicking 
OK/Close until you're back to the Explorer window. Now find a text file, 
right-click on it and you should see your new action. Click it and the text 
file opens in Word instead of Notepad. 

Troubleshooting file types 

Sometimes you'll run into the situation where you double-click a file, but 
instead of the correct associated application opening the file, you'll get 
a small dialog box from Windows asking you what application you want to 
open this file with. When this happens, either you were trying to open a 
file that requires software not installed on your PC yet, or the software 
did not register its file types correctly during installation. A quick way 
to recognize whether a file type is registered or not is by looking at the 
icon next to the file name in Explorer. Is it a colorful custom icon with 
some sort of symbol - it means the file type is registered, or is it a 
white piece of paper with the generic Windows logo in the middle? It means 
the file type is not registered. 

If you are confident that you do own the software required to use this type 
of file, you should first try reinstalling the software. Chances are that 
it will register the file types correctly this time and everything will 
work. 

If that does not help and you get the Open with ... dialog box again, 
scroll through the list of programs, find the correct application, check 
the box Always use this program to open these files. This should make the 
correct association and remember it for the future. The only caveat here is 
that you might not get any custom actions that would have been normally 
configured during the installation. 

If you do not have the software installed, but you have another application 
that you want to use to open this file type, choose that application in the 
Open with ... dialog box to establish that association. 

If you want to open a file with an application other than the default one, 
click once on the desired file, then hold down the Shift key and right-
click on the file. In the resulting menu, you'll see now the option Open 
with ... that you can select to temporarily use another application to open 
this file type. But be careful that you don't check the box Always use this 
program to open these files, otherwise it will use this other application 
from now on as the default. 

Open or save it? 

When you open an attachment in Outlook Express or download a file from the 
Internet with Internet Explorer, you get by default a dialog box asking you 
whether you wish to open this file or save it to disk. In this dialog box 
is also a check box to Always ask before opening this type of file. If you 
accidentally unchecked this box, you won't get this dialog anymore and are 
stuck with whatever action you chose the last time. But you can easily fix 
this and get the dialog back. Open Windows Explorer, go to View/(Folder) 
Options, and select the File Types tab. Find the file type that you have 
the problem with and double-click it. In the resulting dialog box, put the 
check mark back in the box to Confirm open after download. 


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