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Unfortunately, many Windows machines are configured so that they hide the "dot 3" extension after a file name; in other words, the 3 (sometimes 4) letter extension that identfies the type of file does not appear. For instance, a Microsoft Word document might not have ".doc" on the end of the filename, or a text file might not have ".txt" after it, or a Web page might be missing the ".htm" or ".html".
This is easily fixed, and it is important that you fix it, especially if you are going to be making Web pages. Knowing that a file is a .htm can be extremely helpful, especially if you've got an accidental link to the file as a ".html"! Further, seeing the extension can be a big help in preventing a virus infection. The Anna Kournikova virus used an extension that ended with ".shs", a type of file that virtually no one should ever open. Without a visible extension, it would have been impossible to tell just by looking that the attachment was dangerous.
However, Microsoft keeps changing the means by which you can set your computer to reveal the extension, so make sure you use the correct method for your version of Windows.