Windows stores its file types and associations in the Registry. You can dig at them there by using the techniques described in Article 11, but it’s seldom a good idea unless you know exactly what you’re doing. Windows provides tools for viewing and changing file types and associations, so you don’t need to visit the Registry unless you’re trying to create very special effects. When you install a program, the setup routine typically handles the registration of any file types associated with the program. The better setup routines check with you before registering the file types, because they may already be registered to other programs. But more aggressive programs monitor the file types associated with them and try to reclaim them each time you run the program. Audio players particularly MP3 players and video players tend to be the worst offenders on this front, but they’re by no means the only ones; productivity programs can be greedy too. The better programs let you specify whether they should reclaim file types automat- ically and, if so, which file types.

Specifying the File Type of a File

Typically, you specify the file type of a file by adding the appropriate extension or one of the appropriate extensions to it. Most Windows programs use common dialog boxes for Save operations. These common dialog boxes include a Save As Type drop-down list that you use to specify the file type for the file. By default, the program displays the most likely file type in the Save As dialog box. If you don’t explicitly specify the extension for the file, the program adds it. For example, if you save a workbook file in Excel 2007, Excel suggests the Microsoft Excel Workbook file type in the Save As Type drop-down list. If you don’t add an extension to the filename, Excel automatically adds the extension .xlsx, which is linked to the Microsoft Excel Workbook file type.

Finding Out Which File Type a File Is

Your first clue to which file type a file is should be the icon that Windows uses for the file. For example, if Windows shows a stylized Word icon for the file, it’s probably some form of Word document. But icons can be hard to identify, especially when they’re small - and in any case, Windows often uses the same icon for several file types for example, different types of Word documents. To find out which file type a file is, right-click the file in an Explorer window or on the Desktop, and then choose Properties from the context menu. The General page of the Properties dialog box for the file displays its file type. Alternatively, switch the Explorer window to Details view, and then look at the Type column.