I have a Python toolkit tool with a DEFile parameter type and I publish it as a geoprocessing service. Then I created a Web Application Builder geoprocessing widget to access it from ArcGIS Online. You can see in the image below how it gets rendered in the widget. When I select "Upload File", it works as expected allowing the user to select a local file which then gets uploaded to the ArcGIS Server.

But I don't understand how to make the URL selection work. I can see in the network trace that the URL does indeed get sent into the REST service, but by the time it reaches my python code it looks like this:
c:\arcgisserver\directories\arcgisjobs\row_dev\devorga_gptools_gpserver\j1b86d313ca9243ef9318a28bcba24ef0\scratch\1
There is no file by that name on the server. The only thing I can think of is that my parameter type is wrong but I can't figure you what it should be. Any ideas? In reality I would be OK without URL functionality but I can't remove it from the widget UI, so as long as it is there I'd like to have it work correctly.