OverviewFile Associations
File associations tell Windows what actions can be performed on different types of files, what programs should be used to carry out each of those actions, and the paths to where those programs can be found.
Each file association associates a specific file extension with a command to use a program to do something to that file. For example, a file association can tell Windows that when files ending with .doc are double-clicked, they should be opened with Microsoft Word.
The program that is associated with a file extension is known as the default viewer for that type of file. For instance, Notepad is the default viewer for .txt files when Windows is installed.
File associations are stored in the Registry under HKEY_CLASSES_ROOT.
Next: The Registry