Categorizing files on that dialog would be inappropriate I guess, 'cause there are a lot of them. By the way, "Music" is too specific, if I were to decide, I would named it "Media". That's how I do when partitioning
That usage bar looks pretty but I don't get what it'd be used for. I can see one for your entire system: if you need to free space looking at the bar will tell you what is taking up your space. But for individual folders...
Also, it's misleading; there isn't a certain amount of space a folder can use.
Hmm, I'm not 100% sure about the Files/Music distinction. How will you distinguish what types of files are in a directory? Theoretically there could be 20% Photos, 20% Music and 20% Videos, that would result in quite a few nodes/colors (3+2). Or am I missing the point of your mockup entirely?