This is the only information I can find about this:
Attachments Tab
The option to attach files to postings will not be displayed on the board until the Attachments function is fully configured. To do this, you must first create an attachment storage directory on your server, give it full permissions (0777 on Unix), and add the path to this directory to the Attachment Storage Directory field.
Then, include the URL to the directory in the Full URL to Attachment Storage Directory field. Defaults have been set for the Allowed Attachment File Extensions (note that these extensions are not accepted: .php, .php3, .php4, .cgi, .pl, .exe, .bat, .reg) and the Maximum Attachment Size, but you may wish to change these.
