My PC is is set to 'show' hidden file extensions.
I just change the extension from .txt to .tap. Then tell windows it's fine if it shows a warning box and jobs a guddun. Simples.
Not actually tried loading a .txt in mach to be fair. I'll have to try it.
I guess you can try 'load gcode' then change the looked for extension from .tap to 'all files' so you see every file. Then try loading in a .txt file.