code: function handleFileSelect(evt) {
          evt.stopPropagation();
          evt.preventDefault();
  
          // Get the FileList object that contains the list of files that were dropped
          var files = evt.dataTransfer.files;
  
          // this UI is only built for a single file so just dump the first one
          dumpFile(files[0]);
      }
docs: this function gets called once the user drops the file onto the div