code: function (e, data) {
                  if (e.isDefaultPrevented()) {
                      return false;
                  }
                  var that = $(this).data('blueimp-fileupload') ||
                          $(this).data('fileupload');
                  if (data.context && data.dataType &&
                          data.dataType.substr(0, 6) === 'iframe') {
                      // Iframe Transport does not support progress events.
                      // In lack of an indeterminate progress bar, we set
                      // the progress to 100%, showing the full animated bar:
                      data.context
                          .find('.progress').addClass(
                              !$.support.transition && 'progress-animated'
                          )
                          .attr('aria-valuenow', 100)
                          .children().first().css(
                              'width',
                              '100%'
                          );
                  }
                  return that._trigger('sent', e, data);
              }
docs: Callback for the start of each file upload request: