Flatten PDF with Attachments

Hi,

I,

1. render a xdp in browser (via servlet)
2. fill out the form, attach some files
3. submit the form as PDF (via servlet)
4. launch a process that flattens the PDF file

the flattened PDF doesn't include attachments no more, it was lost durning the Output/transform Qpac. (if I don't apply the flattening, I am able to see the attachments)

How do I keep attachments in flattened pdf?

Thanks in advance.