Certain file types are already associated with programs by Windows, program installers or yourself. It means that, for instance, when you double click in Explorer on TXT file, Notepad will load that document, or when you double click on XLS Excel will load that spreadsheet. (BMP, JPG, GIF, CDR, PSD, TXT, DOC, PDF, HLP, XLS, MOD, XM, MP3, MPG, ZIP, URL, HTM...)