File Type Associations
File type associations are the links between your documents and the applications that use them. The most apparent use of this feature is that, for example, Windows knows to open Notepad when you double-click a text document on the desktop or show you an online order form for anatomical enhancement if you click the link in a spam email message.
One might assume that the aforementioned text file somehow knows it’s a Notepad document, but that isn’t the case. Instead, Windows determines how to handle a file based solely on the filename extension. The extension is the group of letters—usually three—that follow the period in most filenames. For example, the extension of the file Readme.txt is .txt, signifying a plain-text file. Likewise, the extension of Resume.docx is .docx, which tells Windows the file is a word processor document in the Microsoft Word file format. See the sidebar, next, for a little detail on this point.