How to do an ajax submission to Craft Contact Form with attachments using FormData?
I have it working now using this:

$('#Form').submit(function(ev) {
// Prevent the form from actually submitting
ev.preventDefault();

var formData = new FormData($(this)[0]);

// Send it to the server
$.ajax('/', {    
  url:  '/',
  data: formData,
  type: 'POST',
  async: false,
  cache: false,
  contentType: false,
  processData: false,
  success: function(response) {
    if (response.success) {
      alert('Thank You. Your message has been sent');
    } else {
            // response.error will be an object containing any validation errors that occurred, indexed by field name
            // e.g. response.error.fromName =&gt; ['From Name is required']
            alert('An error occurred. Please try again.');
          }
        }
      });
});