I'm building a Form in a module using the Form API. I've had a couple of dependent dropdowns that have been working just fine. The code is as follows:

$types = db_query('SELECT * FROM {touchpoints_metric_types}') -> fetchAllKeyed(0, 1);
$types = array('0' => '- Select -') + $types;
$selectedType = isset($form_state['values']['metrictype']) ? $form_state['values']['metrictype'] : 0;
$methods = _get_methods($selectedType);
$selectedMethod = isset($form_state['values']['measurementmethod']) ? $form_state['values']['measurementmethod'] : 0;
$form['metrictype'] = array(
    '#type' => 'select',
    '#title' => t('Metric Type'),
    '#options' => $types,
    '#default_value' => $selectedType,
    '#ajax' => array(
        'event' => 'change',
        'wrapper' => 'method-wrapper',
        'callback' => 'touchpoints_method_callback'
    )
);

$form['measurementmethod'] = array(
    '#type' => 'select',
    '#title' => t('Measurement Method'),
    '#prefix' => '<div id="method-wrapper">',
    '#suffix' => '</div>',
    '#options' => $methods,
    '#default_value' => $selectedMethod,
);

Here are the _get_methods and touchpoints_method_callback functions:

function _get_methods($selected) {
    if ($selected) {
        $methods = db_query("SELECT * FROM {touchpoints_m_methods} WHERE mt_id=$selected") -> fetchAllKeyed(0, 2);
    } else {
        $methods = array();
}
    $methods = array('0' => "- Select -") + $methods;
    return $methods;
}

function touchpoints_method_callback($form, &$form_state) {
    return $form['measurementmethod'];
}

This all worked fine until I added a file field to the form. Here is the code I used for that:

$form['metricfile'] = array(
    '#type' => 'file',
    '#title' => 'Attach a File',
);

Now that the file is added if I change the first dropdown it hangs with the 'Please wait' message next to it without ever loading the contents of the second dropdown. I also get the following error in my JavaScript console:

"Uncaught TypeError: Object function (a,b){return new p.fn.init(a,b,c)} has no method 'handleError'"

What am I doing wrong here?