"Failed to extract" error when icon is not in located in the appdir root

[DetachHead]

thanks for fixing #41. all my appimages (mostly) work now except one:

https://github.com/DetachHead/rebased/releases/download/0.1.6/Rebased-x86_64.AppImage

admittedly this is the one appimage i maintain myself, so i'm not sure how common this pattern is. the icon file is not located within the root directory, but rather in /usr/bin:

[Desktop Entry]
Name=Rebased
Exec=AppRun
Icon=/usr/bin/idea
Type=Application
Categories=Development;IDE;
Terminal=false
StartupWMClass=jetbrains-rebased
StartupNotify=true