The behavior of FormData.append is quite strange

[yarlc]

Version

24.14.0

Platform

Windows 10

Subsystem

No response

What steps will reproduce the bug?

Since fs.openAsBlob doesn't support large files, I decided to rewrite the File.stream function, but the result was unexpected.

class MyFile extends File {
    constructor() {
        super(["1"], "test.txt", { type: "text/plain" });
    }
    stream() {
        return new Response("2").body;
    }
}

async function f1() {
    const file = new MyFile();
    const form = new FormData();
    form.append("file", file, "othername.txt");
    const resp = new Response(form);
    console.log(await resp.text());
}

async function f2() {
    const file = new MyFile();
    const form = new FormData();
    form.append("file", file);
    const resp = new Response(form);
    console.log(await resp.text());
}

console.log(await f1()); // MyFile.stream has not been called, file content is "1"
console.log(await f2()); // MyFile.stream was called,          file content is "2"

How often does it reproduce? Is there a required condition?

always

What is the expected behavior? Why is that the expected behavior?

The expected output of file content will be the same.

What do you see instead?

The content of the file has changed after being assigned a new name.

Additional information

No response

[Renegade334]

This is exactly the behaviour dictated by the spec (https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#create-an-entry).

If a new filename is provided, then a new File object is created with the specified filename and the same byte data as the input. Because this new object is a File, not a MyFile, it doesn't have your custom methods.

If you want your custom File object to be passed through unaltered, you cannot specify a new filename.

[yarlc]

I hope fs.openAsBlob supports large files (>2G).