infrastructure/testdriver/file_upload.sub.html

<!doctype html>
<meta charset=utf8>
<title>File upload using testdriver</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<form id="form">
  <input id="file_input" name="file_input" type="file">
</form>
<script>
promise_test(() => {
  let form = document.getElementById("form");
  let input = document.getElementById("file_input");
  return test_driver
    .send_keys(input, String.raw`{{fs_path(file_upload_data.txt)}}`)
    .then(() =>
      fetch("file_upload.py",
        {method: "POST",
         body: new FormData(form)}))
     .then(response => response.text())
     .then(data => {
       assert_equals(data, "PASS");
     });
});
</script>