Fix inconsistent behaviour when using files widget with other file inputs on the page

Steps to reproduce using Studio:

• build a creation layout with:
• one simple file widget
• one multiple files widget
• chose a file for the first input, and the first element of the second list input
• without submitting the form, remove the first element of the list using the "delete" icon

=> the second chosen file is set on the first file widget instead of its previous value