Android 6.0 Marshmallow introduced an annoying feature last year, which doesn’t let you connect your device to PC in MTP mode directly. And unfortunately, the feature is making to Android Nougat as well in the 2016.
On Nougat, you can’t set Transfer files (MTP) as default mode of connection when connected to PC with a USB cable. What this means is you’ve to select the “Transfer files” option from the notification drop-down each time you connect your device to the PC.
Some might suggest that it’s possible to set Transfer files (MTP) as default on Nougat from Developer options » Networking » Select USB Configuration option, but that’s only going to work for once and as soon as you disconnect your device the connection setting will reset.
So yes, even if you hated/complained about this inconvenient feature on Marshmallow to select MTP from notification drop down each time you want to transfer files between your device and PC, Google still decided to keep it on the Android 7.0 Nougat update.
Anyway, we’re still hoping Google would revise this setting on the final release of Android Nougat in fall. OR else we might be looking at an Xposed module very soon to workaround/fix the damn thing.