Allow multiple file uploads for File questions in native Forms

[netforce-dev]

When migrating from GLPI 10.x using the Formcreator plugin to the native Forms module in GLPI 11, users can no longer attach multiple files to a single File question.

Many service request forms require several supporting documents (PDFs, images, spreadsheets, etc.). The current workaround is to create multiple File questions, which makes forms harder to design and less user-friendly.