Filedocuments are part of the system module and thus can never be used directly. So you always need to create an entity with the Generalization of System.filedocuments or System.Image depending on if you want to work with files or images.
Please note that the documentation you refer to is for Studio, while you are using Studio Pro. The documentation is up to date, but not for the editor you are using ;-)
See the following documentation: https://docs.mendix.com/howto/data-models/working-with-images-and-files
As ronald mentioned, Filedocuments are part of the system module which cant be used directly.
You can follow this documentation
Did you created an object for OrderUploads and did you associated the created object to the OrderSummary entity?.
This issue will come when you didnot create an object for OrderUploads
I hope this helps you
before calling this page, you need to make changes .
First create a microflow which call this page. In the same microflow , created an object for OrderUploads and create an object for OrderSummary entity and associate both of them then call the page by passing OrderSummary object