Allow describing file upload fields

Currently, we allow specifying both filters and validators related to file uploads. However, file uploads need to be of a different `Zend\InputFilter\Input` type in order to be validated correctly: `FileInput`. This patch allows toggling a value indicating whether or not the field is an upload file; if so, the UI will send a `type` for the given field with a value of `Zend\InputFilter\FileInput`. Additionally, any fields returned by the API with that type will display with that flag toggled on.