[Feature Request] Single document download: use basename instead of full storage path in filename

Description Description When using a multi-level Storage Path (e.g. {{ correspondent }}/{{ document_type }}/{{ title }}), the "Use formatted filename" checkbox added in #12095 uses the full relative path on disk and joins the path segments with underscores instead of using just the final segment (the actual filename on disk). Example • File on disk: Acme Corp/Invoice/2026-01 Office Supplies Invoice.pdf • Downloaded filename: Acme Corp_Invoice_2026-01 Office Supplies Invoice.pdf • Expected: 2026-01 Office Supplies Invoice.pdf Related reports of the underlying flattening behavior: #5602 Why this matters Storage paths are commonly used to organize documents into folders on disk, while the actual filename (last path segment) is meant to be the clean, shareable document name. Right now, using folders in the Storage Path makes the single-document download filename unusable, forcing users to choose between disk organization and clean downloads. …