https://tools.ietf.org/html/draft-ietf-nvo3-vxlan-gpe-02#section-10.2

Current implementation is compliant with prior draft but new Draft adds MPLS as type 5

0 = reserved
1 = ip4
2 = ip6
3 = ethernet
4 = nsh
5 = mpls
6...253 = unassigned

Up to community to decide if this needs to be a 16.06 improvement or not but given this appears to have been added to the Nov2015 NVO rev (https://tools.ietf.org/html/draft-ietf-nvo3-vxlan-gpe-01) and no one has complained yet....