I have the same problem: an import will populate “Product Image” but not “Download Files”. I was testing by mapping this image URL to both fields: https://www.army.mil/e2/c/images/2011/10/03/221736/original.jpg.
Your test of putting the image in wp-content gave the same result using URL (http://localhost/wp-content/uploads/…). I also tried various absolute filesystem paths (like /wp-content/uploads/…) but could not get any to work for either field.
I am using EDD 2.6.10 on WP 4.6.1.
