some stuff.

progress.js

  $(document).ready( function() {
    var form = $('#fileUpload'),
        progress = $('#progress'),
        data = new FormData();

    form.submit(function(event) {
      event.preventDefault();

      var file = $('#file')[0];
      progress.text('Starting to upload file:' + file.files[0].name);
      data.append('key', '<%= @key %>/' + file.files[0].name);
      data.append('AWSAccessKeyId', '<%= @id %>');
      data.append('acl', 'public-read');
      data.append('policy', '<%= @policy %>');
      data.append('signature','<%= @signature %>');

      data.append('file', file.files[0]);

      console.log('submit', data);

      $.ajax( {
        processData: false,
        contentType: false,
        type: 'POST',
        url: '<%= AWS_HOST %>',
        data: data,
        xhrFields: {
          withCredentials: true
        },
        dataType: 'xml',
        success: function(response) {
          progress.css('width', '0%');
          progress.text('File upload done. Sending callback.');
          console.log(response);
          $.ajax( {
            type: 'POST',
            url: '<%= @callback %>',
            dataType: 'json',
            data: {upload: {filename: file.files[0].name, key: '<%= @key %>'}},
            success: function(response) {
              progress.text('Finished.');
              console.log(response);
              form.hide();
            }
          });
        },
        xhr: function() {
          // Patch in some progress
          var xhr = $.ajaxSettings.xhr();
          if(xhr.upload){
            xhr.upload.addEventListener('progress', function(event) {
              if (event.lengthComputable) {
                var percentComplete = parseInt(event.loaded / event.total * 100, 10);
                console.log(percentComplete)
                progress.text('File upload ' + percentComplete + '%');
                progress.css('width', percentComplete + '%');
              }
            }, false);
          }
          return xhr;
        }
      });
    });
  });