It seems to be after a "." in general it cuts off the rest, i assume this is because you want to exclude ".mp3", a small change i can remember from my homemade mp3 rename programs (QBASIC) was to use the command right$ and left$, of course this is not the way to do it in QT/Pyhton etc... hehehe
But basically just starting the "remove" process from the end of text first, or just make it check for extension, if ".mp3" is present then remove 4 chars from filename.

Again not critique, just hopefully some help