A way to upload files is something we plan on adding to the form plugin. The dropzone field that is used in the admin is very specific for page content media.
You could write a custom field that did that easily enough though. There are plenty of select type fields throughout Grav that get their selection options from a method. If you look at this example in the system.yaml blueprint:
YOu see this is simply calling a static method on the Utils class to return an array of values. You could either via plugin or theme add a static method that could return the media associated with a page.