We have a Creative ZEN (vendor id 041e, vendor product 4157) that
theoretically supports the MTP protocol: mtp-probe and mtp-detect find
it, and there is a rule for it in /lib/udev/rules.d/69-libmtp.rules.
However, accessing the device through libmtp (e.g. via the gvfs mtp
backend in Nautilus, or with manual mounting via mtpfs) fails to find
any files on the device. The contents of the device can be properly
accessed when using the PTP protocol using libgphoto2.
As a provisional solution I changed the udev rule to use MTP_NO_PROBE
and NOT make the device visible through libmtp, but proper support for
the device in libmtp would be preferred.