r=new window.WHATWGFetch.Response(
  "--0123456789\r\nContent-Disposition: form-data;name=test0\r\nContent-Type: text/plain\r\n\r\nHello1\nHello1\r\n--0123456789\r\nContent-Disposition: form-data;name=test1\r\nContent-Type: text/plain\r\n\r\nHello2\nHello2\r\n--0123456789--",
  { headers: { 'Content-Type': "multipart/form-data; boundary=0123456789"} }
);
f=await r.formData();
console.assert(f.get('test0') === 'Hello1\nHello1');
console.assert(f.get('test1') === 'Hello2\nHello2');
console.log(Object.fromEntries(Array.from(f.entries())));

{test0: 'Hello1\nHello1', test1: 'Hello2\nHello2'}