Enable JSON mime type by default to avoid loose the extension with long filenames

While preparing the unit tests for MDL-62240 I've realised that JSON is not enabled by default. That means that, when exporting data, the extension is lost if the file exceeds the long limit, because JSON is not in the mime types list (so it's not treated as an known extension).

We should add JSON to the default mime types list now that we're using for the privacy export (to make sure all the files preserve the .json extension).