title: The named parameter 'fileExtension' is not defined
body: <p>I am using some code from an older version of File Picker, the null safety version will mess up my other code so I'm using 1.1.1.
  the code isn't incorrect because it the same on the file picker package example. can anyone help plz?
  the code goes as followed</p>
  <pre><code>void openFileExplorer() async {
  try {
    _path = null;
    if (_multiPick) {
      _paths = await FilePicker.getMultiFilePath(
          type: _pickType, **fileExtension**: _extension);
    } else {
      _path = await FilePicker.getFilePath(
          type: _pickType, **fileExtension**: _extension);
    }
    uploadToFirebase();
  } on PlatformException catch (e) {
    print(&quot;Unsupported operation&quot; + e.toString());
  }
  if (!mounted) return;
  </code></pre>
  <p>}</p>