Go into Admin Panel - Settings - Attachments
Check "Allow Attachments"
Check "Allow online attachments upload"
Leave "Authenticated users Only. (User must be logged in to upload files)" unchecked.
Enter the maximum filesize in bytes (or just use the system default)
In "Attachment folder" enter the full path to where you want attachments stored. This directory must be writable (CHMOD to 0755)
Under "Accepted File Types" enter a list of acceptable file extensions, separated by a comma (e.g. '.txt, .doc, .jpg')