**Take note that root access is required.**MTP support has been an essential part of Android™ for many years now.Android™ Marshmallow got many new security features - also in view to USB-connection-to-PC-handling... So enabling MTP in Android™ 6.0 has proven to be problematic. You have to allow MTP or PTP every time, you connect your marshmallow'ed device to PC via USB.
This app makes it as convenient as possible. MTP Enabler allows you to select your desired mode with just a few clicks.