Working out where all your disk space has gone is a perennial puzzler - as is trying to work out where the heck you put those WAV recordings from last Christmas. And Disk File Searcher UWP is well placed to help, though I ultimately found its interface too quirky and confusing. See below for screenshots.
This APP shows a count by file type , grouped by photos , videos, music and documents. It features memory space occupied by each file type , and lets you open files by running the associated application.
So here goes with some screenshots to explain how it presents your phone's (or laptop's - this is a full Windows 10 UWP app) storage:
The UWP app and its hamburger menu - it's all quite well laid out and wonderfully... orange! (right) some helpful tips included, though it's never entirely clear when a search is running as to whether you need to search again afterwards for something different. I was left confused several times.
Reporting on file types found, here showing 38 WAV files, then (right) reporting on where they were found. Right, let's dig down...
Tapping into the folder shows the files and can be opened to see what they are, etc; (right) in Settings you can pick the file types scanned for.
There's quite a bit that I'd like to take up with the developer, not least that stepping 'back' one too many times exits back to the phone Start screen. I kept having to go back into the app over and over again! Look, if a user wants to go back to Start in Windows phones then they HIT THE START navigation control! Plus I then got 'stuck' on the root of my microSD card and couldn't get 'back'. Oh well. Something for an update, I think?
Anyway, you can grab a free trial in the Store here. This is ad-supported and then the idea is that you remove the ads with an IAP if you find it useful enough.