code: function isValidDrop(items) {
          var i, len = items.length;
  
          for (i = 0; i < len; i++) {
              if (items[i].kind === "file") {
                  var entry = items[i].webkitGetAsEntry();
  
                  if (entry.isFile) {
                      // If any files are being dropped, this is a valid drop
                      return true;
                  } else if (len === 1) {
                      // If exactly one folder is being dropped, this is a valid drop
                      return true;
                  }
              }
          }
  
          // No valid entries found
          return false;
      }
docs: Returns true if the drag and drop items contains valid drop objects.
  @param {Array.<DataTransferItem>} items Array of items being dragged
  @return {boolean} True if one or more items can be dropped.