GridStore reference by filename not working

The feature of the GridStore to lookup a file by it's name just didn't work. This patch should fix that.

As a side note: maybe this feature should be removed AT ALL. Since file names don't have to be unique, the behavior is a bit unpredictable. For a read operation you can't know for sure whether the requested file is correct, for a write operation it's not obvious if you are creating a new file, or appending / overwriting an existing one...