code: function scanFolders(rootFolderPath, folders, folderFileExtensions, callback) {
          var result = [];
  
          console.log('Scanning:: Folders', folders);
          console.log('Scanning:: Root folder path', rootFolderPath);
          var fileExtensions = self.fileExtensions.slice();
          scanFolder();
  
          function scanFolder() {
              var folder = folders.shift();
              if (folder === undefined) {
  //                console.log('Scanning:: Finished with result', result);
                  return callback(null, result);
              }
              if(folderFileExtensions.length){
                  fileExtensions = folderFileExtensions.shift();
              }
              //var path = rootFolderPath + '/' + folder;
              var path = rootFolderPath + folder;
  
              console.log('Scanning:: Full path to folder', path, fileExtensions);
  
              fs.readdir(path, function (err, list) {
                  if (err) {
                      console.error("Error scanning folder: ", path);
                      return scanFolder();
                  }
  
                  var files = [];
  
                  for (var i = 0; i < list.length; i++) {
                      if (acceptFile(path, list[i], fileExtensions)) {
                          //files.push((folder.length ? folder + '/' : folder) + list[i]);
                          files.push(path + '/' + list[i]);
                      }
                  }
  
                  result = result.concat(files);
  
                  scanFolder();
              });
          }
      }
docs: Private functions 
  Gets files from folders
  
  @param rootFolderPath
  @param folders {Array}
  @param callback
  
  @return {Array} array of files