If you're wanting to use appender just make an appender and replace the ~= to calls to appender.put(data);
Just making Subdata an Appender!(string[][]) (or Appender!(Tuple!(string, string)[])) is enough since it already overloads the ~= operator.
Performance aside, a small nitpick is that it's possible to write filter!isFile instead of filter!(a => a.isFile) since isFile only accepts one argument.
