code: @Nonnull
    public FineUploader5Session setCustomHeaders (@Nullable final Map <String, String> aCustomHeaders)
    {
      m_aSessionCustomHeaders.setAll (aCustomHeaders);
      return this;
    }
docs: Any additional headers you would like included with the GET request sent to
  your server. Ignored in IE9 and IE8 if the endpoint is cross-origin.
  
  @param aCustomHeaders
  Custom headers to be set.
  @return this