The attached image GWG181_16Bit_CMYK_X4.pdf from the Ghent Workgroup test suite is not displayed, PDFBox throws the mentioned exception. One open source and one closed source product display an "X", but gswin renders the image properly.
The upcoming patch handles the 16bit case. I won't implement 1, 2 or 4 bpc because I don't have test images.
I'll add my patch to 1.8 after the cut.