I have cleaned up some 2.6 GB junk today from Internal memory

My mobile had an internal memory of 16 GB , out of  which, 6 GB was ' system reserved ' . When I checked ' File Manager ' yesterday , Internal Memory was occupying a whopping 8 GB and only 2 GB left .

I checked Apps . They were not taking a lot of space . I cleaned up all ' Sent pictures , Videos ' etc . Transferred all videos onto External Memory card . Cleaned all Whats App junk . Tranferred all that could be transferred to External Memory card . Still no use. It's still reading 7.9 GB Internal Memory .
So I connected Mobile to PC . To find any extra files that I  could not find directly on Mobile.