You have added these as an enum. In the schema alternate hash functions are done as a choice of different elements. Admittedly at the moment the only valid choice is HashSHA1 - see https://www.openmicroscopy.org/Schemas/Documentation/Generated/OME-2012-06/ome_xsd.html#Plane_HashSHA1 

This makes sense from a schema point of view as it can check the value matches the right type.