made method for guessing file extension public

Change-Id: I827429eac70d6d5b4bb0bd47c8ca5433b3df00e4
Reviewed-on: http://gerrit.esdi-community.eu/3563
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>
Tested-by: Simon Templer <simon.templer@igd.fraunhofer.de>