[GEOS-6686] MBTiles and GeoPkg are not correctly shown in LayerPreview

Conflicts:

	src/community/mbtiles/src/main/java/org/geoserver/mbtiles/gs/wps/MBTilesProcess.java
Merge pull request #771 from n-lagomarsini/GEOS-6686_2_6

[GEOS-6686] MBTiles and GeoPkg are not correctly shown in LayerPreview (Backport on 2.6.x)