By default the Simple Machines forums are set to allow only very small attachments, and the max folder size is set very low as well.

If you ever get an error that the upload file is full, you can increase the max upload size per post and max folder size (that holds all the uploads) in Admin -> “Attachments and Avatars” (under Forum). For OLI Forum Hosting customers, we recommend setting the “Max attachment folder space” to 200000 kb (which is 200mb) and “Max attachment size per post” and “Max size per attachment” to 50000 kb (which is 50mb).