In what? Roundcube or on the forum?
In RoundcubeCreate a new message.
Click the "
- " button under the "Attachments" title
Find your file and upload....
In the Forum- Click either "Reply" (in the tabs) or type your post and hit "Preview" under quick reply box.
- Click the "Additional Options..." link
- Click the "Browse..." button to attach your file (valid file types are listed below input box).
Hope that answers your question.