Android Phone taking up more space than it should

I have a relatively new HTC ONE M8 running Android 5.0.1 (according to the settings page). The phone is supposed to have 10.5 GB space according to Windows, but at the moment only 945 MB is free (again, according to Windows). However, when I select all of my files within and check properties it shows that they all take up only 5 GB of space, meaning another 4-5ish has disappeared. Any ideas how to fix this, as I'm trying to free up space for more music and pictures? Screenshots from DiskInfo and DiskUsage: