It's just a txt file really, with no extension... i expect i created mine on my pc & copied it over, but you could create it with nano directly on the phone if you have it installed... Having nano installed is a good idea 'cause chances are you will need to edit your credentials when last.fm has a security breach, but when does that ever happen ... http://www.bbc.com/news/technology-18358485

I attached mine (credentials removed). You need to take the extension off to use it - upload here seemed to need the extension on. Edit it with wordpad on PC or nano on the phone