So, just as question states. Android device monitor is not showing anything in the File Explorer view. This is only occuring on Virtual Devices running Android API Level 24
A temporary solution, which worked at least for my needs, was to install file manager .apk (I used Astro file manager .apk) - just drag/drop .apk to emulator window. Another very useful utility - ImDisk - which let's mount sdcard .img of virtual device. If you mount it as readonly ejectable device, you can have realtime access to virtual device file system