mechanize：Mechanize::Form::FileUpload#mime_type

everyday Ruby

以下のような HTML で

<html>
<head>
<title>form test</title>
</head>
<body>

<form method="get" action="cgi/foo00.cgi" name="form1">
  <input type="file" name="File" />
</form>

</body>
</html>
>> require 'mechanize'
=> true
>> agent = Mechanize.new
=> #<Mechanize:0xb74d85ac ...
>> page = agent.get('http://127.0.0.1:8080/test12_form.html')
=> #<Mechanize::Page
  ...
>> f = page.form('form1')
=> #<Mechanize::Form
  ...
>> f.file_upload_with(:name => /File/).class
=> Mechanize::Form::FileUpload
>> f.file_upload_with(:name => /File/).mime_type
=> nil

POST したときに設定されるのかな？いや、逆に、設定して submit すりゃいいのかな？