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)
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.
Emad's Blog 
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