How to modify a process variable or a bussines variable, from the activity form, witout using the contract?

Hi all

I am a newbie at Bonita 7.2.0, and have some doubts.

I have added a new porcess variable in the Bonita Studio, and I want to modify the value from a form button, I have one contract in that task to send datas from the form to the bonita flow.
In the button that I use to send the task, I have a javascript expression like this, with ti objects from the contract:

Handle Multiple Documents

Hi, I'm designing a form that will enter information through a user, will have other fields to upload multiple documents at once, you can add one or more, but these documents are optional, the form must move forward even if the user does not upload files . While load the file advances to the next stage, however, the problem is when any file is not added and not let me move to the next task.