A little tip from the dark ages
Do not bother to scrape, file, emery, or do what ever to get the chrome plating off the tube, just get the smallest amount of hydrochloric acid you can buy, the measure precisely the depth of the socket of the fitting, with a container plastic for preference, it helps if the container is the same depth as the socket of the fitting, fill the container with the acid then holding the chrome plated tube vertically dip the tube into the acid then into cold running water, the chrome will have gone, there might be nickel plating, but that wont hurt, now you can soft solder the fitting to the copper or use a compression fitting