File Block: use inner block for Button

[creativecoder]

What problem does this address?

As mentioned in #52850, the File Block button is a kind of orphan in term of styling. It will inherit global button colors, but otherwise can't be style from the Global Styles UI.

What is your proposed solution?

Update the File Block to use a inner Button Block. This way Button Block styles can be used to modify the File Block's button.