I just recently downloaded some music from Amazon(first time) when first downloaded it had a file ext. I had never seen before.  I went back to it this morning to find the extension is now .mp3 and this for some reason it wont play in my or my wife's car both are 2011 models. what ext. should I be using so I can burn in the house and listen in the car.

My Computer is XPS 8700 mid 2013

stock CD player and LG  BD and a whole host of other things it can do.

What do I do?

Oh the OS is win HP