Lets say I have a .thescriptergeek file. Not even set up in registry, no program to edit it, etc. Could I treat it as if a .txt file?
Sure, the os will ask you how to open it and you can choose a text editor.
If you create a program you are not bound to the file extensions.
Like @coder777 said, file extensions don't matter at all for C++ (e.g. using fstream). The file extension is just treated as part of the file name with fstream.