Improve admin upload error for Vercel 4.5 MB function body limit

Describe the Feature

When a Payload app is deployed on Vercel and media uploads are sent through the server upload path, files larger than Vercel Functions' request body limit fail with a raw 413 Content Too Large / FUNCTION_PAYLOAD_TOO_LARGE network error.

It would be helpful if the Payload admin upload UI could detect this failure and show a clearer message with file-size context, for example:

Upload failed: this file is 5.1 MB, but this deployment can only accept about 4.5 MB through the server upload route. Enable client uploads for your storage adapter, or upload a smaller file.

Use Case

In a Payload 3 + Next.js + Vercel deployment using @payloadcms/storage-vercel-blob, uploading a 5.1 MB PDF to /api/media?depth=0&fallback-locale=null failed online with:

POST https://example.vercel.app/api/media?depth=0&fallback-locale=null 413 (Content Too Large)

The same file worked locally, and the Payload config had a larger upload limit, so the root cause was not obvious from the admin UI.

The fix was to enable direct client uploads:

vercelBlobStorage({
  collections: { media: { disablePayloadAccessControl: true } },
  clientUploads: true,
  token: process.env.BLOB_READ_WRITE_TOKEN,
})

Proposed Improvement

When an admin upload request receives HTTP 413, display a specialized upload error that includes:

• the selected file size in MB
• the server/provider response status, e.g. 413 Content Too Large
• guidance that hosted/serverless providers may have lower request body limits than Payload's configured upload.limits.fileSize
• for Vercel Blob / cloud storage adapters, a hint to enable clientUploads

This would make the problem understandable without needing to inspect the browser console/network tab.

Related Context

• Vercel Functions document a 4.5 MB request/response body limit.
• @payloadcms/storage-vercel-blob already documents that clientUploads bypasses this limit.
• Related prior discussion: https://github.com/payloadcms/payload/discussions/7569
• Related prior issue: https://github.com/payloadcms/payload/issues/8231

Start at the admin upload UI and the /api/media request path, then trace how HTTP 413 responses are surfaced. Check the existing upload error handling and the related Vercel Blob clientUploads documentation. Done means a 413 upload displays the file size, provider status, and guidance about serverless limits and client uploads.