Q: Display won't sleep and auto-sleep doesn't work

Never had problem like that before. So I read some advices. I did reset of SMC and NVRAM not even once... I switched off internet sharing, file sharing. I tried unplug USB HDD/SuperDrive/Hub/USB Keypad, and plug it back. I tried to reset printing system.  I reset settings through System Preferences - Energy Saver. Nothing works.

When I set pmset -g assertions it says:

2016-08-05 04:06:32 -0500

Assertion status system-wide:

   BackgroundTask                 0

   ApplePushServiceTask           0

   UserIsActive                   0

   PreventUserIdleDisplaySleep    0

   PreventSystemSleep             0

   ExternalMedia                  1

   PreventUserIdleSystemSleep     0

   NetworkClientActive            0

Listed by owning process:

   pid 61(powerd): [0x00008f7000081490] 00:13:25 ExternalMedia named: "com.apple.powermanagement.externalmediamounted"

Kernel Assertions: 0xc=USB,BT-HID

   id=510  level=255 0x8=BT-HID mod=12/31/69, 6:00 PM description=com.apple.driver.IOBluetoothHIDDriver owner=BNBMouseDevice

   id=511  level=255 0x8=BT-HID mod=12/31/69, 6:00 PM description=com.apple.driver.IOBluetoothHIDDriver owner=AppleBluetoothHIDKeyboard

   id=513  level=255 0x4=USB mod=12/31/69, 6:00 PM description=com.apple.usb.externaldevice.14100000 owner=USB2.0 Hub            

   id=516  level=255 0x4=USB mod=12/31/69, 6:00 PM description=com.apple.usb.externaldevice.14500000 owner=USB3.0 Hub            

   id=517  level=255 0x4=USB mod=12/31/69, 6:00 PM description=com.apple.usb.externaldevice.14120000 owner=USB Keyboard

   id=518  level=255 0x4=USB mod=12/31/69, 6:00 PM description=com.apple.usb.externaldevice.14140000 owner=USB2.0 Hub            

   id=520  level=255 0x4=USB mod=12/31/69, 6:00 PM description=com.apple.usb.externaldevice.14540000 owner=USB3.0 Hub            

   id=521  level=255 0x4=USB mod=12/31/69, 6:00 PM description=com.apple.usb.externaldevice.14600000 owner=External USB 3.0

   id=522  level=255 0x4=USB mod=12/31/69, 6:00 PM description=com.apple.usb.externaldevice.1d130000 owner=IOUSBHostDevice

   id=523  level=255 0x4=USB mod=12/31/69, 6:00 PM description=com.apple.usb.externaldevice.1d131000 owner=UVC USB2.0 Camera

Idle sleep preventers: IODisplayWrangler

KeyReels-Mini:~ keyreel$

What is actually IODisplayWrangler? I had issues with auto-sleep before, I had issues with computer sleep, but with with Display sleep, that is the first time and it drives me crazy.

Manual sleep works finally without issues.

Please help! Thank you

MacMini 2012 10.11.6, Apple Cinema Display 23