I'm not sure on the structure of the snippet/sample. Shouldn't it be something more like (pseudocode)

return CanDeserialize(body) ? new MessageWrapper() : null

or

try
{
  return new Deserialize(body);
}
catch
{
  return null;
}

I'm concerned about the focus on checking for base64 (an internal implementation detail) rather than making it clear that if the message can't be handled by the custom unwrapper, then null should be returned.