An AC Reactive Reference can be rendered as formatted HTML. This allows you to render a parameter as a variety of HTML control types. In addition, if the HTML rendered is of the format <input type="InputType" name="value" > the value returned from the parameter is usable in the build.
This works successfully with at least the following InputTypes that I've tested:

• checkbox
• range
• month
• datetime-local
• color
  That's pretty cool, but I now want to render a Jenkins File Parameter.

The groovy script for this is very simple:

html='''
<input name="value" type="file" jsonaware="true">
'''
return html

This is rendered and functions correctly with one exception. The selected file never gets uploaded to the server as a normal Jenkins File Parameter would. Making this work would be a great way to render a dynamic File Parameter in the UI.