It may be because you are still using a very old version of the framework - 1.67. Try the latest version, which is 1.91.
This is what I have in one of my tasks to display an "Add Attachment" button.
In the _cm_changeConfig() method I have the following:
$this->fieldspec['add_attachment'] = array('type' => 'string',
'control' => 'input',
'task_id' => 'py_email_attachment(add2)');
$fieldarray['add_attachment'] = null;
In my language_text.inc file I have the following:
$array['add_attachment'] = "Add Attachment";
The XML output contains the following:
<add_attachment name="task#py_email_attachment(fileupload)" control="input">Add Attachment</add_attachment>