xlsx files were download-only. Add a read-only, virtualized preview (sheet
tabs, number formats, merges, theme colours) parsed entirely in a browser
Web Worker with exceljs and fflate, loaded only when a spreadsheet is
opened. The workbook is checked against ZIP-bomb, entry and cell limits
before exceljs loads; cell text is written with textContent, formulas are
never evaluated and nothing referenced by the workbook is fetched. On the
server xlsx only joins the existing allowlist and classification, with a
10 MB cap on ?preview=true. xls and ods stay download-only.
Aamer Akhter