gnome-disks truncates a huge image when restoring to a drive from xz-compressed image file

This bug affects Lubuntu Xenial i386 but not amd64.

gnome-disks works when restoring from a 300 MB image compressed with xz. But it fails when trying to 'restore' from a 12 GB image compressed with xz. This extraction works with mkusb, which uses xzcat and dd under the hood. The process is described at

https://help.ubuntu.com/community/Installation/UEFI-and-BIOS

and the file to 'restore' is

http://phillw.net/isos/linux-tools/uefi-n-bios/dd_Ubuntu_16.04-gamma-UEFI-n-BIOS-4-pendrive-12GB.img.xz

gnome-disks 'thinks' that the file size is 3.5 GB, so it truncates it, and the result is a corrupted system on the target drive. See the attached screenshot.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: gnome-disk-utility 3.18.3.1-1ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-18.34-generic 4.4.6
Uname: Linux 4.4.0-18-generic i686
ApportVersion: 2.20.1-0ubuntu2
Architecture: i386
CasperVersion: 1.372
CurrentDesktop: LXDE
Date: Sat Apr 16 21:26:16 2016
LiveMediaBuild: Lubuntu 16.04 LTS "Xenial Xerus" - Beta i386 (20160415)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-disk-utility
UpgradeStatus: No upgrade log present (probably fresh install)