FormData.append() stringifies Blob/File — fetch with a FormData body sends 0 bytes and no content-type #9842

Summary

FormData.append() stringifies every value, so a File or Blob is stored
as the string "[object Object]". Consequently a Request/fetch with a
FormData body sends zero bytes and no content-type.

Per the spec, append(name, value) stringifies only when value is not a
Blob; a Blob/File must be kept as an entry value.

This is the outgoing half of multipart. The incoming half is fine — the
Request.formData() fix from #9617 works correctly, verified separately below.

Environment

Repro

const bytes = new Uint8Array(1024);
const file = new File([bytes], "rad.jpg", { type: "image/jpeg" });
const blob = new Blob([bytes], { type: "application/octet-stream" });

const fd = new FormData();
fd.append("f", file);
fd.append("b", blob);
fd.append("t", "hello");

console.log(fd.get("f") instanceof File);   // expected true
const req = new Request("http://x/", { method: "POST", body: fd });
console.log(req.headers.get("content-type"));            // expected multipart/form-data; boundary=…
console.log((await req.arrayBuffer()).byteLength);       // expected > 2048

Observed:

File before append   typeof=object  isFile=true   isBlob=true   size=1024  name=rad.jpg
Blob before append   typeof=object  isFile=false  isBlob=true   size=1024
fd.get('f')  [File]  typeof=string  ctor=String   isFile=false  isBlob=false  str="[object Object]"
fd.get('b')  [Blob]  typeof=string  ctor=String   isFile=false  isBlob=false  str="[object Object]"
fd.get('t')  [text]  typeof=string  ctor=String                               str="hello"
keys=["f","b","t"]
Request content-type: null
Request body bytes: 0 (expect > 2048)

So:

1. File/Blob construct correctly — size, name, type and instanceof
   are all right before append.
2. append stores them as "[object Object]".
3. new Request(…, { body: fd }) sets no content-type and yields a
   0-byte body. fetch does the same, so the server sees an empty request.

Note constructor.name is null for a correctly-constructed File/Blob,
which may or may not be related.

The incoming side is fine

Hand-crafting a multipart body and parsing it on the server works exactly as
expected on this same build — so #9617's fix is good and this is a separate
defect:

formDataKeys: ["file","caption"]
 file    → isFile=true  isBlob=true  name="a.bin"  size=9  mime="application/octet-stream"
 caption → typeof=string  "hello"

Impact

Any client that uploads with FormData sends an empty body. It fails
quietly — the request is accepted, the server simply sees no file. In our
case (MB24) that is listing photos, KYC documents and claim photos.

It also makes a FormData-based test unable to exercise a working server
parser, which is what masked the state of #9617 for us: our spike posts with
new FormData(), so it still failed after the parser was fixed.