Nexus 6 file transfer over USB is not an option I have a Nexus 6 (just factory reset).  When I connect it to my computer through USB, it charges, but nothing else.  My computer doesn't recognize any new device being plugged in, and my phone does not pop up with the dialog asking what I want to do (Charge, MTP, PTP, etc.), nor do I get a notification asking this, even when pulling down from the notification drawer.  I am running Android 6.0.  I have another Nexus 6, and it works fine.  When I plug the first Nexus 6 into any computer, I have the same issue, while the second device works fine in any computer.  I have tried Windows and Mac computers.
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)