I have found a great site if you are having trouble with file extensions.
The files listed here are all ZIP files, which contain a REG (Registry) file. Download the ZIP and open it. Extract the REG file to your hard disk and double click it. Answer yes to the import prompt. REG files can be viewed in Notepad. Each of the REG files contains the default settings for the file extension indicated. For the ZIP file fix, the download is a REG file, since ZIP’s aren’t working anyway!