Hey there! What a strange occurrence haha I'm almost positive it's happening because you changed it so .cfg files open in notepad! There are two possible fixes, the second fix is somewhat lengthy but shouldn't be too difficult if you're familiar with the way your computer runs and what not.
Fix 1
Run your command prompt and provide the following command: assoc. .exe=exefile
Fix 2
Step 1: Open your windows command prompt and type in cd\windows
Step 2: This activates your windows folder! Now type in regedit
Step 3: Scroll all the way to the top of the registry and find where it says "HKEY_CLASSES_ROOT and double click it
Step 4: Now scroll down until you find the .exe Look to the window in the right and double click where it says "Default". This will open a small window, you will see a box that's title "Value Data". Type "exefile" into that box
Step 5: Now scroll waaaay down until you find a folder with a dropdown titled "exefile". Within the dropdown go to shell, then open, then command.
Step 6: Once in command, look to your right window and double click "Default". Under Value Data type "%1" %* Make sure you include the quotation marks in this one.
Step 7: Click okay and restart your windows and you should be all fixed!!