Fails to read MBTiles with JPG file format

As initially reported on StackExchange:

https://gis.stackexchange.com/questions/270770/geoserver-cant-add-a-jpeg-mbtiles-file

The spec confirms "jpg" is a valid format here:

https://github.com/mapbox/mbtiles-spec/blob/master/1.3/spec.md

As suggested by Andrea and Ian, the fix is to add "JPG" to the enum here:

https://github.com/geotools/geotools/blob/master/modules/unsupported/mbtiles/src/main/java/org/geotools/mbtiles/MBTilesMetadata.java#L43

GitHub pull request incoming.

(Credit to my co-worker Murray McDonald for tracking this down and verifying the fix)