[BUGFIX] Extension keys must be lowercase

Change-Id: I3c5a2bb7e292c0ada8d7f1f73d3ba29607843255
Resolves: #57759
Reviewed-on: https://review.typo3.org/29806
Reviewed-by: Tomas Norre Mikkelsen
Tested-by: Tomas Norre Mikkelsen
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Oliver Klee
Reviewed-by: Thomas Löffler
Tested-by: Thomas Löffler