public static boolean isValidMapFile(final Uri filePath) {
        return getAttributionIfValidFor(filePath) != null;
    }
checks whether the given Uri is a valid map file.
Thie methos uses cached results from previous checks
Note: this method MUST be static because it is indirectly used in MapsforgeMapProvider-constructur
main/src/main/java/cgeo/geocaching/maps/mapsforge/MapsforgeMapProvider.java