When you create a custom display form in SharePoint Designer for a Custom list, you dont' see list item attachments if it has any.
To show the list item attachment in display format , place the following control in customdisplayform.aspx using designer.
<SharePoint:AttachmentsField ControlMode="Display" FieldName="Attachments" runat="server" Visible="true"/>
*customdisplayform.aspx is my display form for custom list.