Retina Sass Mixin

// RETINA MEDIA QUERY
// '@include background-image-retina('foobar', 'png', 10px, 20px);'

@mixin background-image-retina($file, $type, $width, $height) {
  background-image: url($file + '.' + $type);

  @media (-webkit-min-device-pixel-ratio: 2), (-moz-min-device-pixel-ratio: 2) {
    & {
      background-image: url($file + '@2x.' + $type);
      -webkit-background-size: $width $height;
    }
  }
}