This directory needs to contain the files

  icon_128x128.png
  icon_128x128@2x.png
  icon_16x16.png
  icon_16x16@2x.png
  icon_256x256.png
  icon_256x256@2x.png
  icon_32x32.png
  icon_32x32@2x.png
  icon_512x512.png
  icon_512x512@2x.png

On OSX, they are obtainable by executing the following command:
iconutil -c iconset /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/GenericDocumentIcon.icns
