I have a friend at another tech forum who also has a NAS as I do. He installed the My Media server on his PC and after some fiddling around got My Media to stream music from his NAS to the Echo. Problem is, the computer must be on, whereas a NAS does not require a computer to be on.
He does not want to use My Media because he (and I) have ways to stream music to Alexa directly from NAS, but for those who do not have a NAS, then using My Media to stream from a computer (or attached drive) to Echo may find My Media to be a good way to go..