I would like to access the file from which the following is read off.

CodePudding user response：

File types/associations are stored in the HKEY_CLASSES_ROOT part of the registry.

Which application (which ProgId technically) is the default for a specific file extension is stored inside the undocumented HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts key. The UserChoice is hashed to prevent people from changing the default.