There is no word to be written - you have to tell MP3tag which character shows that a word ends and the next starts. So it is just a list of single characters.
If your filename does not change the case then you have a filesystem that does not really care - you would have to rename the file so that there is a real modification to the filename like an added underscore or something.
And all this would be much, much easier if you filled the tags and then created the filename from the tag fields.
Modifying the filename is something for dedicated file handlers.