Inky tries to export JSON as a .ink file

When exporting the ink story to JSON, inky by default tries to export it as a .ink file instead of a .json file, to avoid this you have to type that you want it to be a .json file, but it should be done by default.

No file, test, or entry point is named in the issue. First reproduce an ink story JSON export in Inky and trace the export filename handling; done means JSON exports default to a .json filename without requiring manual extension input.

Written by the indexing model from the issue text.