Mimetype detection should take place for every mix:mimeType node not just nt:resource

ModeShape's mime type detection mechanism is triggered atm. only when a node is nt:resource and not more generically when it has the mix:mimeType mime type.

The code should be changed so that if a node has the mix:mimeType mixin the mime-type detection capability is triggered and the mimeType property value set on the node.