If I select ".json" in the file open dialog, no JSON files are filtered. Just save e.g. a file called "test.json" in a directory and then try to File -> Open... and select "*.json|GeoJSON" as filter.
Seems to work fine with latest Marble KDE master.
@Torsten: Was this fixed by quickgit.kde.org/?p=marble.git&a=tree&hb=aff3557 ? If so we should backport that patch. I tested the Qt version of Marble and it works fine there.