How to modify document list content

Hi,
Can you help me? (I'm using Bonita Community 7.2)

I've a process involving various steps:

From Instantiation form I acquire Model Contract and a variable list of files as attached documents via upload widget:

Please note that some file are mandatory and some optional!

My Process Instantiation Inputs are:

Please note: "allegatiRANFInputs" type FILE is "MULTIPLE"!!!

Where my Pool variables are:

And my Documents are:

Defined as follow:

Please note "Initial content->From contract".

This is my "formOutput.allegatiRANFInput" with 3 attachment (2 mandatory and 1 optional) before submit in JSON format:

This is H2 bonita_journal.db content after submit:

Now, in a subsequent human task: "Modifica la richiesta rifiutata", I need to present to user the list of previously attached files (done via a check list) and the user must have the choice to change some of them. The "Task Inputs" are:
- richiestaANFInput:COMPLEX
- allegatiRANFInput:FILE:MULTIPLE

I have:

That I show to user as follow:

If user uncheck the checkbox from existing attachment then it can be upload as new file:

Then user will upload only the removed mandatory document and in formOutput.allegatiRANFInput' I have:

Now if in the operation pane of human task if I use the default operation "Set document list":

It works but I lost the 2 existing documents (that user do not want do modify and that do not upload again) and list was updated and contain only with the new uploaded document.

So I removed the previous op. "Set documetn list" and I defined a fake string variable "aggiornaATTACH" that take values "":

from the following script:

But does not work!

Which is the correct way to "update document list" in bonita without removing existing ones?

Thanks