In my forum, for the users with higher than writer authority can attach files(lower than writer they do not see attach file menu) but I want to authorize registered users and contributor to be able to attach files.
Can you please let me know how I can grant them?
Please read the documentaion. All registered users are able to attach a file by default.
You just need to manage Antispam settings in Dashboard > Forums > Tools admin page. Just set the min number of posts "0" for "Min number of posts to be able attach files" option.