From Online Manual
Attachments are files or images attributed to a post. Commonly, attachments are used when the file or image is directly related to the post and the user has no way to put the file online elsewhere. Each post can have multiple attachments, as defined by the Administrator.
Attachments are stored on the server the forum resides on, and uses the disk space provided. Because of this uploaded attachments are often limited in size, or width and height, to prevent overuse of disk space.
Administrators can choose what type of files attachments can be, and disable undesired file formats. Administrators can set up various limits and restrictions to meet their needs with the Attachments and Avatars Manager in their Administration Center.
By default, the number of attachments that can be posted on a forum is limited to 4,294,967,296. In the unlikely event that the limit is reached, it can be increased fairly easily.