We normally use SharePoint Designer to edit the default forms of a list and add a Custom List form web part in the SharePoint site. This will work fine. But when we try to add an attachment, by clicking Attach File in the custom form you will receive this error message “This form was customized not working with attachment“..
I found out that Microsoft has released hot-fix 953749 to address this issue along with a recommended resolution for fixing the XSL to allow attachments to work again. You can get the details here. The resolution details how to address all form types (new, edit and display). There is even a recommended fix for the error “An unexpected error has occurred.”