Publishing Excel Services to TMG

Hi guys, I'm having problem with my sharepoint 2010 being published to TMG 2010, all works except that the excel services isn't displaying the chart or image, the problem is the server hostname is being changed when the image is being downloaded so the tendency is to fail. I have published my sharepoint with external hostname sharepoint.domain.com but when its downloading the chart/image for excel report the hostname is changed to sps2010 which is the internal name of the sharepoint server. All works if i access it in the internal network using the sharepoint's internal hostname.

Regards,

Arnel Gracela