How do I load an .atf file into a ByteArray for upload to the GPU?

I'm trying to optimise the amount of GPU memory my textures use for a Stage3D project so I thought I'd finally switch to ATF textures. I've converted them from png to atf using the png2atf tool and I'm loading them with a URLLoader but I'm having trouble passing the textures into ByteArrays. I've tried various things and I'm running out of ideas; Does anyone know the AS to pass a loaded atf into a ByteArray? I need it to be a ByteArray for upload to the GPU with the

uploadCompressedTextureFromByteArray method on the Texture class - unless someone knows better...