No AppRun extracted when using --appimage-extract on a type 2 AppImage

Using Ubuntu 16.04 running in the WSL (on Windows 10 Fall Creators Update), passing `--appimage-extract` on a type-2 AppImage generated using [linuxdeployqt](https://github.com/probonopd/linuxdeployqt) extracts all files *except* the AppRun file.

The AppImage used in the log below can be found inside this attachment:
[Godot-Linux-x86_64.zip](https://github.com/AppImage/AppImageKit/files/1407532/Godot-Linux-x86_64.zip)

```
squashfs-root/.DirIcon
squashfs-root/AppRun
TODO: Implement inode.base.inode_type 10
squashfs-root/godot.desktop
squashfs-root/godot.svg
squashfs-root/usr
squashfs-root/usr/bin
squashfs-root/usr/bin/godot
squashfs-root/usr/lib
squashfs-root/usr/lib/libFLAC.so.8
squashfs-root/usr/lib/libX11-xcb.so.1
squashfs-root/usr/lib/libXau.so.6
squashfs-root/usr/lib/libXcursor.so.1
squashfs-root/usr/lib/libXdamage.so.1
squashfs-root/usr/lib/libXdmcp.so.6
squashfs-root/usr/lib/libXext.so.6
squashfs-root/usr/lib/libXfixes.so.3
squashfs-root/usr/lib/libXinerama.so.1
squashfs-root/usr/lib/libXrandr.so.2
squashfs-root/usr/lib/libXrender.so.1
squashfs-root/usr/lib/libXxf86vm.so.1
squashfs-root/usr/lib/libasyncns.so.0
squashfs-root/usr/lib/libcrypto.so.1.0.0
squashfs-root/usr/lib/libdbus-1.so.3
squashfs-root/usr/lib/libfreetype.so.6
squashfs-root/usr/lib/libglapi.so.0
squashfs-root/usr/lib/libjson-c.so.2
squashfs-root/usr/lib/libnsl.so.1
squashfs-root/usr/lib/libogg.so.0
squashfs-root/usr/lib/libpng12.so.0
squashfs-root/usr/lib/libpulse-simple.so.0
squashfs-root/usr/lib/libpulse.so.0
squashfs-root/usr/lib/libpulsecommon-4.0.so
squashfs-root/usr/lib/libsndfile.so.1
squashfs-root/usr/lib/libssl.so.1.0.0
squashfs-root/usr/lib/libvorbis.so.0
squashfs-root/usr/lib/libvorbisenc.so.2
squashfs-root/usr/lib/libwrap.so.0
squashfs-root/usr/lib/libxcb-dri2.so.0
squashfs-root/usr/lib/libxcb-dri3.so.0
squashfs-root/usr/lib/libxcb-glx.so.0
squashfs-root/usr/lib/libxcb-present.so.0
squashfs-root/usr/lib/libxcb-sync.so.1
squashfs-root/usr/lib/libxshmfence.so.1
squashfs-root/usr/share
squashfs-root/usr/share/icons
squashfs-root/usr/share/icons/godot.svg
```