Nexus 6 file transfer over USB is not an option
You should try some of the following:


Make sure your phone is not encrypted. (Settings>Personal>Security>Encrypt Phone).
Try enabling USB debugging in dev settings and use adb devices and see if it is listed (you will obviously need adb).
Keep trying USB cables!
Try using something like ASUS File Manager to wirelessly transfer the files.
If they are photos you are trying to transfer, consider using Google Photos (accessible at drive.google.com)