The answer is in the warning you get 'If you change a file extension, the file may becomr unusable. Are you sure you want to change it?' - Changing the extension of a file doesn't change the file format. eg. If you have file1.doc and rename it to file2.mp3 it's still a .doc file.
It won't open in word because the extension is mp3, and it's not an mp3 file. If you want to rename files you should only change the part in front of the extension eg file1.doc to file2.doc