From Online Manual
Attachments are documents or images attributed to a post. Attachments are stored as files within the forum website. They give members a place to store important files for as long as they are needed. In Attachments and Avatars, administrators can choose which file types to allow, who can upload attachments, and can set limits on file size and number of attachments to conserve disk space. By default, the number of attachments that can be posted on a forum is limited to 4,294,967,296, but this can be increased if necessary.