Drupal 7: File field causes error with Dependable Dropdowns - drupal-7

Question

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?