Users cannot attach files
Users do not have the option to attach a file, although placing the permissions as administrator does not appear.
Are you using Auto optimise or some kind of optimization plug in?
wpForo Spam Control has an option to restrict new registered users to attach files. There is a level (minimum number of posts) they must have to see File Attachment button. You can set this option value 0 in Dashboard > Forums > Tools > Antispam > New Registered Users section. Just set the minimum number of posts to be able attach a file “0”.