javascript upload string blob as file

upload_file.js

function upload(txt) {
  // wrapper by Blob
  const blob = new Blob([txt], { type: "text/html" });
  // warpper by File
  const file = new File([blob], "test.plan");

  const formData = new FormData();
  formData.append("File", file);

  fetch("http://localhost/upload", {
    method: "POST",
    body: formData,
  })
    .then(async (res) => {
      const txt = await res.text();
      if (res.ok) return txt;
      else throw new Error(txt);
    })
    .then((success) => console.log(`upload success: ${success}`))
    .catch((err) => console.error(`upload failed: ${err}`));
}