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chromium/third_party/blink/web_tests/wpt_internal/forms/file/file-input-show-picker.html 

<!DOCTYPE html>
<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>
<title>INPUT TYPE=FILE and showPicker()</title>
<input type="file">
<script type="module">
import {mockFileChooserFactory} from '../resources/mock-file-chooser.js';

promise_test(async _ => {
  const file = document.querySelector('input');
  const promise = new Promise(
      resolve => mockFileChooserFactory.addEventListener('open', resolve));

  await test_driver.bless('show picker');
  file.showPicker();

  const openEvent = await promise;
  assert_equals(openEvent.detail, 'kOpen');
}, "File picker: showPicker() should show picker");
</script>