PDF missing from upload MIME fallback map

Describe the Bug

Payload's upload MIME fallback map does not include .pdf, so when file type detection from the buffer returns no result, a PDF filename falls back to text/plain.

That can cause valid PDF uploads to fail with:

File type text/plain (from extension pdf) is not allowed.

This was observed with Payload admin client uploads using @payloadcms/storage-vercel-blob.

Reproduction Context

A media collection allows PDFs explicitly:

upload: {
  mimeTypes: ['image/*', 'application/pdf', 'video/*'],
}

Uploading a PDF through the Vercel Blob client upload flow submits metadata to /api/media after the direct Blob upload. In the server-side upload validation path, fileTypeFromBuffer(file.data) can return no detected type, then getFileTypeFallback('file.pdf') returns text/plain because pdf is not in extensionMap.

I checked payload@3.84.1, and dist/uploads/getFileTypeFallback.js currently maps common text/image-ish extensions but not pdf:

const extensionMap = {
  css: 'text/css',
  csv: 'text/csv',
  htm: 'text/html',
  html: 'text/html',
  js: 'application/javascript',
  json: 'application/json',
  md: 'text/markdown',
  svg: 'image/svg+xml',
  xml: 'application/xml',
  yml: 'application/x-yaml',
}

Expected Behavior

.pdf should fall back to application/pdf, so the existing application/pdf validation and PDF corruption check can run.

Suggested map addition:

pdf: 'application/pdf'

Environment

• Payload: confirmed in 3.80.0 and 3.84.1
• Storage adapter: @payloadcms/storage-vercel-blob
• Deployment target: Vercel

Start with dist/uploads/getFileTypeFallback.js and trace it to the corresponding source entry point. Reproduce the server-side upload validation path described for a PDF whose buffer type is undetected; done means the fallback resolves the PDF MIME type so the existing PDF validation can run.