The "file" in project can't open¶

• in a new system ,install redmine2.6,and can login used whitout "File" in Project

• when look for "FILE" in PROJECT ,the web return "500" for me
  
• look for the log in redmine,"TypeError (no implicit conversion of nil into String):
  app/controllers/files_controller.rb:34:in `index'"
  
  NOTE:Started GET "/projects/department-construction/files" for 192.168.2.26 at 2016-01-22 18:26:02 +0800
  Processing by FilesController#index as HTML
  Parameters: {"project_id"=>"department-construction"}
  Current user: admin (id=1)
  Completed 500 Internal Server Error in 15.1ms
  TypeError (no implicit conversion of nil into String):
  app/controllers/files_controller.rb:34:in `index'

• HOW TO USE THE "FILE" SUCCESS