Add support for ISO files

[cewbdex]

So the iso gets mounted automatically and chooses the wim file.

[jhoff80]

Personally, I'd also like to see the file picker not force a file called "install.wim". I mean, I get it, by default it should be called that. But the picker should also have the option to show all files... so that I can find 1809-arm64-install.wim, for example. (In case it wasn't clear from my example, that's a file name I chose to differentiate multiple install.wim files I have saved.)