handle-files-frontendlisted

# Handle files in a frontend app Centralize file work in **one `file.util.ts`** (+ a media/upload service), not ad-hoc per feature. Examples React/TS; principle → **▸ Example** → **▸ Other stacks**. ## 1. CSV import/export Export JSON → CSV with a CSV lib (e.g. papaparse `unparse`) + the download helper; import with `parse`. ```ts export const downloadCsvFromJson = <T>(rows: T[], fileName = 'download') => download({ content: unparse(rows), contentType: 'text/csv', fileName }); ``` ## 2. Download (single + batch zip) One `download({ content, contentType, fileName })` wrapper over `file-saver`; batch many files into a zip (`jszip`) for multi-download. "By URL" / "by id" variants resolve the blob, then save. ```ts await saveMultipleFilesAndZip(files, 'export.zip'); // jszip + file-saver ``` ## 3. PDF View with a PDF component (e.g. react-pdf) — set up the **pdf.js worker once**. Read page count via pdf.js for validation/pagination: `getPdfPageCountFromUrl(url)`. ## 4. Upload — sniff MIME, presign, resize - **Detect the real MIME from file CONTENT (magic numbers), not the extension or `file.type`** (both lie / are spoofable): ```ts async function detectMimeFromContent(file: File): Promise<string> { const b = new Uint8Array(await file.slice(0, 12).arrayBuffer()); // match signatures: PNG 89 50 4E 47 · JPEG FF D8 FF · PDF 25 50 44 46 · GIF 47 49 46 · ... } ``` - **Upload via a presigned URL** — the server signs, the client POSTs the blob straight to stora