The first time one clicks a file with an extension in Windows 8 - windows 10, it will prompt for user choice like this dialog in the screenshot to ask which program one wants to use to open this type of file. if one does not make a choice, it will keep asking one to make a choice the next time of user clicking. if one wants to change the program for this file type, one can right click and then choose the menu "open with" and then select another program, or one can change the default program for the kind of file association in the control panel. With any of the 2 methods one will be able to get the same dialog prompting for user choice. The question is, how can one make windows behave as if one had not made a choice before? I have done the research the whole night, the story is not stored in the registry, I have compared every registry entry of my 2 PCs, they are both the same for the association of file extension. Anyone who has figured out where is this setting located, please do tell me. I think it may be in the "programData" of %windir% or %appdata%/roaming.