it is not possible!

changing icon of a text file (*.txt) is totally difference of changing the ICON of a EXE file or a folder.

you can not change the icon of a single text file.
it is only possible to change the icon of *.txt type files using registry keys. it means that the icon of all files with *.txt extenssion will be changed but it is dependent to the system you have channged it if you move to other system the icon of the *.txt files will base on the setting of that system in the registry.