Custom SharePoint Forms with Attachments

There was a known problem when trying to add attachments to a customized SharePoint forms. that caused an error saying “This form was customized not working with attachment"

Microsoft has "fixed" this issue with the SharePoint Services infrastructure updates (http://support.microsoft.com/kb/953271 (archived copy))

Please note the following to make this fix work:

Make sure to change ALL instances of ControlMode to "Edit" in case of the Edit form and "New" in case of the new Forms.
It is NOT enough to hide the Original un-customized web part, you may need to CLOSE it.