Problem with a custom form and "file" component

Hi there,

First of all, thanks a lot for an awesome module! It is very simple, lightweight, and I have found it perfect for our needs.

I have only ran in one issue: I have a simple custom form, with just one file input field. Something like this:

function myform() {
	$form['file_picture'] = array(
		'#type' => 'file',
	);
	$form['submit'] = array('#type' => 'submit', '#value' => 'Upload picture');
	$form['#attributes']['enctype'] = 'multipart/form-data';
	$form['#ajaxsubmit'] = TRUE;
	return $form;
}

When I press the submit button, it displays a "an error occurred" javascript alert. If I just change "type" from "file" to "textfield", then it works fine.

Has this been noticed before? (I couldn't find any reference to it)