message: Don't lose base thumbnail when converting from zip.xml
diff: @@ -911,6 +911,11 @@ static bool ZIP2Buffers(const std::string &filename, ThumbTypeAndData &thumb,
  extThumb.format = type;
  extThumb.data.assign(buf, sz);
  }
  + else
  + {
  + thumb.format = type;
  + thumb.data.assign(buf, sz);
  + }
  }
  else
  {
  