Bug: Bun server loses File MIME type when reading request body as Blob

[dinwwwh]

What version of Bun is running?

1.3.14

What platform is your computer?

Linux 6.17.0-35-generic x86_64 x86_64

What steps can reproduce the bug?

When a File object with a custom MIME type (e.g. application/json;charset=utf-8)
is sent as a POST body, calling request.blob() on the server returns a Blob whose
.type is reset to text/plain;charset=utf-8 instead of preserving the original type.

Affected: Bun (tested on latest)
Expected: blob.type === 'application/json;charset=utf-8'
Received: blob.type === 'text/plain;charset=utf-8'

import * as assert from 'node:assert'
import { sleep } from 'bun'

let file: Blob

const server = Bun.serve({
  async fetch(request) {
    file = await request.blob()

    return new Response('OK')
  },
  port: 0,
})

const res = await fetch(`http://localhost:${server.port}`, {
  method: 'POST',
  body: new File(['"name"'], 'file.json', { type: 'application/json;charset=utf-8' }),
})

await sleep(1000) // delay between client and server

console.log(file!.type)
assert.equal(file!.type, 'application/json;charset=utf-8')
// Expected: "application/json;charset=utf-8"
// Received: "text/plain;charset=utf-8"

What is the expected behavior?

No response

What do you see instead?

No response

Additional information

No response

[robobun]

✅ Reproduced on 1.4.0: request.blob() in a Bun.serve handler dropped the request Content-Type (every type fell back to text/plain;charset=utf-8).
Fix in progress → #32806