How to access file via selected table object.

Hey guys,
I'm new to bonita and today I tried the upload widget by using this example:

So I can upload in one task a pdf-file and download it again in the next task. That works.
But now I want to download the file in another way. My project looks pretty simple, I have 2 forms and one Page.

Issue in retrieving documents through contract and executing document upload script as connector


This question is in reference to and a follow-up of the query I had posted earlier on this link.

So I have a simple doc upload form that has a list of documents and an file upload widget with each.
In my contract, I have defined a variable of type file that accepts multiple objects --> formDocuments.
In my UI designer I have a similar JSON that takes files that are uploaded through the form.

Complex type contract

I am using the upload widget in UI designer to upload a photo and save it to a document that I have previously defined. I've made a complex contract that contains a child "upload" of type File. When I run the workflow instance, I get the following error: