appimagetool mis-recognizes some pngs as i386 binaries

`appimagetool` mis-recognizes some pngs as i386 binaries.

To reproduce, run on a deb-based system:

```
wget "https://raw.githubusercontent.com/AppImage/pkg2appimage/master/pkg2appimage"
bash -ex pkg2appimage Bitwig-Studio

(...)
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/lib/x86_64-linux-gnu/libavdevice.so.53.2.0 used for determining architecture x86_64
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/lib/x86_64-linux-gnu/libavcodec.so.54.35.0 used for determining architecture x86_64
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/lib/x86_64-linux-gnu/libasyncns.so.0.3.1 used for determining architecture x86_64
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/bin/resources/ControllerScripts/akai/MPK49.png used for determining architecture i386
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/bin/resources/ControllerScripts/reloop/KeyFadr.png used for determining architecture i386
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/bin/resources/ControllerScripts/reloop/KeyPad.png used for determining architecture i386
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/bin/resources/ControllerScripts/nativeinstruments/Komplete Kontrol/KompleteKontrolS.png used for determining architecture i386
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/bin/resources/ControllerScripts/nativeinstruments/Komplete Kontrol/KompleteKontrolS49.png used for determining architecture i386
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/bin/resources/ControllerScripts/nativeinstruments/Komplete Kontrol/KompleteKontrolS61.png used for determining architecture i386
/home/me/Bitwig-Studio/Bitwig-Studio.AppDir/usr/bin/resources/ControllerScripts/nativeinstruments/Komplete Kontrol/KompleteKontrolS25.png used for determining architecture i386
(...)
More than one architectures were found of the AppDir source directory "./Bitwig-Studio.AppDir/"
A valid architecture with the ARCH environmental variable should be provided
e.g. ARCH=x86_64 appimagetool ...
```

__Workaround:__ Use the [experimental Go-based `appimagetool` replacement](https://github.com/probonopd/go-appimage/blob/master/src/appimagetool/README.md) after having run the above; to complete the job:

```
wget -c https://github.com/$(wget -q https://github.com/probonopd/go-appimage/releases -O - | grep "appimagetool-.*-x86_64.AppImage" | head -n 1 | cut -d '"' -f 2)
chmod +x appimagetool-*.AppImage
VERSION=3.1.3 ./appimagetool-*.AppImage ./Bitwig-Studio/Bitwig-Studio.AppDir/
```

Likely I will not fix this in the "old" `appimagetool` but rather begin to move pkg2appimage over to the [new Go-based `appimagetool`](https://github.com/probonopd/go-appimage/blob/master/src/appimagetool/README.md).