the "file type" or "extension" is the last bit of stuff after the "." in a filename
many programs are notorious for seizing control (.pdf opens with adobe, then foxit reader)
there are a few ways to try and force the OS to actually open it the way you want...
first, right click on a file of the desired type (e.g. .c) Open With -> Choose Program -> Select the Program from a List
Check the checkbox "always use the selected program..." and then click Browse to find the executable that YOU want to open that extension type. (e.g. notepad2.exe)
that can SOMETIMES fix the problem but just as often it has no effect...
instead, right click the file.extension and choose Properties Click on the "Change" button to change which application should open that extension, the menu with Browse will look almost the same as above... and doesn't always work.
To REALLY force the OS, in windows file explorer, Tools -> Folder Options -> File Types
Search through the very long list (which is sorted alphabetically by extension) and you can choose to Change (which looks like the menus above and doesn't do much).
ADVANCED however will actually show you what the OS has as the icon and commands for that extension.
Changing the icon involves finding another image (usually .exe's store an icon image) Editing an existing "open" command is usually the easiest:
Under the "Application used to perform..." you'll find the path to the exe and the parameters passed to the executable, e.g.
"C:\minefield\firefox.exe" -requestPending -osint -url "%1"
The %1 merely tells the application that the file you clicked is going to be loaded by that .exe
If you simply change the path to the executable your extension will be modified! Windows then replaces your "Advanced" button with a "Restore" button which will allow you to change it back to the defaults (but you should copy and paste/screenshot a backup of all default settings before changing it, don't rely on the OS...)
When you have to create a "NEW" action
You actually just choose to Browse to the new executable and you don't have to include the %1...
(DDE doesn't need to be activated, it's a bit advanced to activate cross application data sharing right now, we just want .c to open with notepad2!)
You could create a whole new "action" type (like Edit) but I'm not sure how your click will tell the application the difference between an "open" and an "edit"...
When done with all of these changes you can now double click your file.extension and have it open with the application you desire.