As I need this one day: [WayBack] Just in case I need this again… >Free library DSiWin32 … now implements this as DSiRegisterUserFileAssoc and DSiUnregisterUserFileAssoc <… – Thomas Mueller (dummzeuch) – Google+
Note file has moved from WayBack: gpdelphiunits.googlecode.com/svn/trunk/src/DSiWin32.pas to [WayBack] OmniThreadLibrary/DSiWin32.pas at master · gabr42/OmniThreadLibrary · GitHub because Google code has shut down.
There still is [WayBack] Google Code Archive – Long-term storage for Google Code Project Hosting: gpdelphiunits, but maintenance is now part of [WayBack] GitHub – gabr42/OmniThreadLibrary: A simple and powerful multithreading library for Delphi
Related: [WayBack] delphi – How to associate a program with a file type, but only for the current user? – Stack Overflow
–jeroen