docs: function getDropEffect(event, ignoreDataTransfer) {
          var effects = ALL_EFFECTS;
          if (!ignoreDataTransfer) {
            effects = filterEffects(effects, event.dataTransfer.effectAllowed);
          }
          if (dndState.isDragging) {
            effects = filterEffects(effects, dndState.effectAllowed);
          }
          if (attr.dndEffectAllowed) {
            effects = filterEffects(effects, attr.dndEffectAllowed);
          }
          // MacOS automatically filters dataTransfer.effectAllowed depending on the modifier keys,
          // therefore the following modifier keys will only affect other operating systems.
          if (!effects.length) {
            return 'none';
          } else if (event.ctrlKey && effects.indexOf('copy') != -1) {
            return 'copy';
          } else if (event.altKey && effects.indexOf('link') != -1) {
            return 'link';
          } else {
            return effects[0];
          }
        }