media values of screen and print do upload and are processed with no problem.

prepress will be rejected. After adding an optimization pass with rghost (and obviously ghostcript in the background) it does then upload. I also tried the Foxit PDF Editor, applying only the cleanup optimization and it also did something which made the format good for Kindle Publishing.

Understanding that finding a very small thing in the PDF generation that could be slightly different could be a huge effort, I would propose that a small hint be added to the documentation, pointing that in case of any problem with the prepress media mode, an optimization pass can iron things out.

Best regards