code: public String filePath(int index, String savePath) {
          // not calling the corresponding swig function because internally,
          // the use of the function GetStringUTFChars does not consider the case of
          // a copy not made
          return savePath + File.separator + fs.file_path(index);
      }
docs: returns the full path to a file.
  
  @param index
  @param savePath
  @return