The "Download and import file attachments" option fails while importing a WXR file that references media hosted on an IPv4 private network (10.x.x.x, 172.16-31.x.x, & 192.168.x.x) even if both WordPress servers are hosted on the same network.
Message I'm receiving is
Failed to import Media "Term Name"
The problem appears to be in the WordPress function wp_http_validate_url() which is returning False for me on line 501 inside wp-include/http.php instead of passing through the $url value.
I apologize if this is more of a WordPress Core problem than an Importer Plugin problem but I thought I would start here first.