code: function handleShowInOS() {
          var entry = ProjectManager.getSelectedItem();
          if (entry) {
              brackets.app.showOSFolder(entry.fullPath, function (err) {
                  if (err) {
                      console.error("Error showing '" + entry.fullPath + "' in OS folder:", err);
                  }
              });
          }
      }
docs: Show the selected sidebar (tree or workingset) item in Finder/Explorer