Better $_FILES support

[BenRutlandWeb]

Is your feature request related to a problem? Please describe.
The Request object doesn't handle files gracefully

Describe the solution you'd like
A nice API to handle file uploads

Describe alternatives you've considered
Using the $_FILES array

Additional context
The WP_REST_Request has a get_file_params method and set_file_params method, but these don't format the files into a nice consistent array. Laravel/Symfony map files into a File or UploadedFile object which would allow for some cool fluent APIs to be performed such as store and storeAs.