Dear Maintainer,

I just tried to install Debian Jessie using the current debian-installer for Jessie.
I went through the installation and created a partition layout like this:

   /dev/vda1   2GiB  Swap
   /dev/vda2   40GiB JFS2

I have other installations with Debian Wheezy that work fine with this and installation
progressed normally. However, finishing up the installation and trying to install grub
to /dev/vda, I got a fatal error "unknown filesystem".

Entering the newly installed system from the chroot, the issue seems to be that update-grub
fails because the call 'grub-probe --target=fs -d /dev/vda2' fails with 'Unknown filesystem'
instead of returning the correct value.

# file -Ls /dev/vda2
/dev/vda2: JFS2 filesystem image, 79713504 blocks, blocksize 4096
# grub-probe --target=fs -d /dev/vda2
grub-probe: error: unknown filesystem.

I would have expected to be able to just install grub with the JFS as root device, as it did
quite a few times before.