It doesn't make a lot of sense to limit the listing
to assets/Uploads/, which is the default set through FileField->folderName.
Showing all files regardless of folder makes them easier
to find, users can still opt-in to filtering by folder
through the TreeDropdownField.
DataObject::validate() is currently set to protected, but this means
you can't call validate() from outside the context of itself unless
you overload the method to use a public visibility and then call
parent::validate()
As it would turn out, most classes that overload this method already
set the visibility to public, so it would make sense the parent matches
that as well.
