feat: Inline preview for spreadsheet attachments (.xlsx / .csv)

Feature request: inline preview for spreadsheet attachments

Is your feature request related to a problem?

When a user attaches a .xlsx (or .csv) file to a document it renders as a plain attachment widget (file icon + name + size). There is no way to view the spreadsheet content without downloading the file and opening it in a separate app.

Only PDF attachments currently support an inline preview (shared/editor/nodes/Attachment.tsx renders a PdfViewer when preview is set), so spreadsheet support is a natural extension of that path.

I searched existing issues (excel, xlsx, spreadsheet, csv preview) and could not find a duplicate feature request.

Describe the solution you'd like

• Uploading/dragging an .xlsx (and ideally .csv) file should offer an inline preview of the sheet content, similar to the existing PDF preview.
• Rendering should happen client-side (e.g. SheetJS / exceljs) so it works in fully offline / intranet deployments and does not depend on an external service such as the Microsoft Office online viewer.
• The preview should degrade gracefully back to the current attachment widget on failure or for other office formats.

Describe alternatives you've considered

• Microsoft Office online viewer iframe (view.officeapps.live.com) — rejected because it requires external network access, which self-hosted / intranet installs do not have.
• Pasting cells as a native table — works for manual data but not for files already attached to a document.

Additional context

• Extend the preview attribute path already used by PDF attachments.
• A custom node or plugin (see the plugins/ directory) could host this to keep the core small.
• Happy to submit a PR if the approach is agreed.