This question already has an answer here:
After installing some programs, they appear in the "Set Your Default Programs" list. Selecting one of these and clicking on "Choose defaults for this Program" brings up a list of all extension and internet protocols known to the program.
Where exactly in the registry is this information stored? In other word, how does Windows know which programs to put into the initial list, and which file types and protocols to put into the second one?
(Note: This is the inverse of this question.)