Understand and work with file formats

Understanding extensions
The ability to understand and distinguish between different file types before opening them is useful. If someone sends you a spreadsheet file that won't open, knowing how to identify the file format could solve the problem, and that is where file extensions come into play.

A file extension is the short string of letters and/or numbers that appear after the full stop in a file name. For example, if you have a Microsoft Word document saved as a file called Myfile.doc, then the '.doc' part is the file extension. At a glance you can tell that the file is a Microsoft Word document, because of the .doc extension. Anyone can change the file extension, or use a different one to start with, but most of the time the file extension is a very good indicator of the file type and, thereby, the content.

Before you can exploit this rule of thumb, you need to see the file extensions, but, in an arguably misdirected ease-of-use effort, successive versions of Windows have kept them hidden from users.

So, to turn on the display of file extensions in Windows XP, open My Computer (from the Start menu), select Tools from the menu followed by Folder Options. Windows Vista users should click Start, select Computer, click the Organize button and choose Folder and Search Options.

Now, for XP or Vista, in the Folder Options dialogue box click to bring the View tab to the front. Scroll down the list of options displayed below the 'Advanced settings' heading and click to remove the tick from 'Hide extensions for known file types'. Click OK. From now on, files in Windows Explorer will be shown with their file extensions in place.

Missing extensions
However, changing this setting won't always allow you to tell what format a file is, or which program should open it. We don't wish to labour this point because most of the time the file extension will be accurate, but there may be situations where a file arrives without an extension, leaving Windows at a loss as to what to do with it. Even if the file extension is correct, double-clicking the file to view or edit the content will sometimes launch an unexpected application.

We will consider the former scenario first. For historical and technical reasons Apple Mac computers are not concerned with file extensions. Some Mac programs add common file extensions but they are not needed by the operating system, so you could receive a file from a Mac user that lacks an extension.

In this situation, it is probably easier to ask the person who sent it which application the file is intended for. If they say 'Microsoft Word' then rename the file on your PC, adding a '.doc' extension. In XP or Vista, left-click the file in question and press F2, or right-click and select Rename from the pop-up menu. Press the right cursor (arrow) key once, or click at the very end of the file name, and then type in the missing extension – '.doc', for instance.

Changing associations
However, you don't have to add an extension to open such files. Indeed, you can force Windows to ignore a file's extension and display the contents in an application different from the one it would otherwise use, or permanently change the application it launches in response to double-clicking a particular file type.

This behaviour is controlled by file associations that rely on file extensions. In other words, by default Windows associates the DOC format with Microsoft Word. So, when the user double-clicks on a file with a .doc extension, Windows associates it with Word and launches the application.

As before, this works reliably but there may be times when it fails to work the way you want. For example, if you have just installed a new program it may have taken the executive decision to reassign one or more file associations to itself. Image-editing programs are particular offenders.

To alter the way this works in XP you need to return to Folder Options. As a reminder, open My Computer (from the Start menu), select Tools and then Folder Options. Now click the File Types tab, scroll down the list under 'Registered file types' to find and click the relevant file extension. Now click the Change button and use the Open With dialogue box to pick an alternative program to associate with the file type. If you want to make this change permanent, make sure the 'Always use the selected program...' checkbox is ticked.

The process is slightly different in Vista: right-click any file of the relevant type and choose Open With from the pop-up menu. Now, either pick one of Vista's suggested alternatives or select the Choose Default Program option to browse for a different program. Again, to make the change permanent tick the 'Always use the selected program...' box.

Most of the time you needn't be too concerned about file formats and extensions: double-click a file and it will launch the appropriate application; or, when you save, the application will choose the most suitable file format for the content type.

However, while this automation works well there will be times when you will want your computer to behave differently. The skills demonstrated in this back-to-basics guide will allow you to take control, ensuring that you can open files lacking extensions and alter file associations if necessary. So, though you may not need these techniques today, remember them for the day when you do.

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