I have a folder with 100 tunes in it, all with the extension .m4a. I need to change them to .m4r. They are ringtones for my iPhone. I can do them one by one, going into "get info" and changing the "a" to an "r' but want to do the whole lot somehow in one fell swoop.....they do not need any conversion, simply changing the letter will do. Any suggestions?