File associations are managed in the HKEY_CLASSES_ROOT hive of the registry.
In HKEY_CLASSES_ROOT, there is one key per file extension.
Each key has a default value corresponding to the file type.
For example, let’s looking at the .docx file extension.
Here the default value for .docx is Word.Document.12
In HKEY_CLASSES_ROOT, there is also one key per file type.
All these keys have the following sub-keys:
The default value of the sub-key “command” defines the command run when the file is launched.