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.

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.

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):

  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 storage, so the app never holds storage credentials. Route image uploads through a resize subfolder when needed:

  const contentType = await detectMimeFromContent(file);
  const { url, fields } = await mediaService.getPresignedUrl({ contentType, isPrivate, subFolder });
  await fetch(url, { method: 'POST', body: toFormData(fields, file) });

• Validate type + size before upload; surface failures through the app's error handler (write-frontend-code error-handling section). ▸ Other stacks: same shape — one file module; content-sniff the MIME; presigned-URL upload; swap papaparse/jszip/file-saver/pdf.js for your ecosystem's equivalents.

Verification

• File logic lives in one util module (+ an upload service), not scattered per feature.
• Uploads detect MIME from content (not file.type) and go through a presigned URL (no storage creds client-side); images route through resize when needed.
• CSV export/import and zip-download go through the shared helpers.

Related

• write-frontend-code — services + error handling · structure-a-frontend-app.