code: function(imageFile,mimeType) {
                  // Convert it.
                  var ret = 'data:';
                  ret += mimeType;
                  ret += ';base64,';
                  ret += imageFile.toString('base64');
                  return ret;
              }
docs: Given the contents for an image, returns a data URI string
  representing the data in that image.
  Adapted from:
  https://github.com/jbkirby/css-img-datauri-stream/blob/master/index.js