Describe the bug

When a file name is given to a File object and attached to a Request through a FormData object, the file name given is lost.

I've tracked the issue down to this line of code

where it doesn't copy over the file name if there is one. It is likely that this line was intended to only execute this code for Blobs, but File is a subclass of Blob.

Reproduction

test("file name preserves through a request", async () => {
    const fileName = "my-doc.pdf"
    const file = new File([new Uint8Array(1_024)], fileName, {
        type: "application/pdf",
    })
    const formData = new FormData()
    formData.append("file", file, fileName)
    const request = new Request("http://example.com", {
        body: formData,
        method: "POST",
    })
    const result = await request.formData()
    expect(result.get("file").name).toEqual(fileName)
})

System Info

System:
    OS: Linux 5.15 Ubuntu 24.04.3 LTS 24.04.3 LTS (Noble Numbat)
    CPU: (8) x64 AMD EPYC 7R13 Processor
    Memory: 27.18 GB / 30.67 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 24.12.0 - /home/coder/.nvm/versions/node/v24.12.0/bin/node
    npm: 11.6.2 - /home/coder/.nvm/versions/node/v24.12.0/bin/npm
  npmPackages:
    @vitejs/plugin-react: ^5.1.2 => 5.1.2 
    @vitest/coverage-v8: ^4.0.15 => 4.0.15 
    vite: ^7.2.7 => 7.3.0 
    vitest: ^4.0.15 => 4.0.15

Used Package Manager

npm

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