File Extensions

October 8, 2009

A file is a bucket of information stored on your computer. That file can contain all sorts of different types of information. And the information inside the file can be organized in many different ways. You can tell what type of file it is by the file extension. The term file extension makes it sound like you are extending a file and making it bigger. But the file extension is the last part of the file name after the period. For example, if you saw a file called readme.txt, you would know that the file is a simple text file. That’s because .txt means it’s a simple text file.

When you are looking at files in Windows, it may not show you the file extension. Instead, it will tell you the type of file it is in the “Type” column. In the above example of readme.txt, it would show the file as readme and then in the Type column it would say “Text Document”. There is a setting you can change so that it always shows the file extension, so if you always see the extension, that’s why.

Some file extensions are standard. The .txt file extension used in the example above is a standard. However, there are some file extensions that are used by many different programs. The format of these files varies from program to program. For example, if you see a file with .dat on the end, that could mean just about anything. But don’t worry, most people will never have to worry about these non-standard file extensions.

Windows keeps a list of file extensions. For each extension, it has a designation of what program should be used to open that type of file. These are called file associations. Which program is associated with which file extension, basically. So if you were to double-click on readme.txt, Windows would go to the file association list and see that it should use a program called Notepad to open a file with an extension of .txt on it.

When you rename a file, if you change the file extension, you might get a warning message from windows that says something like, “If you change a file name extension, the file may become unusable”. This can be a scary message if you don’t understand what it means. It doesn’t mean that the file will become corrupted and ruined. It means that when you try and open the file, windows will use the wrong program to open it.

So when you rename a file, be careful not to change the file extension. This is why it’s best for most people to have the setting set so that it doesn’t always show the file extension. That way, when you rename the file, you won’t accidentally change the file extension.

Here are some important file extensions to know about.

A file with .exe, .bat, .com, .msi, and .vbs, among others, are files that will execute commands or programs on your computers. Never open one of these unless you know exactly what it is and where it came from. Why? Because these are the files that could infect your computer.

Another file to be careful with is one with a file extension of .reg. These files contain Windows Registry entries and they can infect your computer or change the configuration of your computer.

While we are on the subject of file names, let’s talk about what is called the full file spec. Have you ever seen something that looks like this:

C:documents and settingsownermy documentslettertomom.doc

This is like an address for a file on the local computer. “C:” is which drive it’s on. “documents and settingsownermy documents” is the path to where the file is located on the drive. “lettertomom” is the base file name and, as you know, “.doc” is the file extension that tells us what type of file it is.

Have you seen something like this:

\fredspccdocuments and settingsownermy documentslettertomom.doc

This is very similar to the previous example with one difference. This one begins with \fredspcc instead of “c:”. When it begins with “\” that tells Windows that this is which computer it’s on. That means Windows is to find the file on a different computer on the network. The name of the computer to look on is “fredspc”. Of course, fredspc has to be set up to allow this type of access, but assuming fredspc is set up to allow access over the network and that file is in a location where security allows you to access it, you could access that file on fredpc this way.

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