Fatal error on multiple custom field import

At

civicrm/import/custom

I get

CiviCRM_API3_Exception: "'Import Multi value custom data' is not a valid option for field mapping_type_id"

This is a fairly recent regression due to a code change that assumed that the contract between mapping_type_id & actual types was respected. I propose to fix but creating the mapping_types on the fly in the import classes. As far as I know only this class is affected but it feels safe to me & by putting in it's own function this doesn't seem crazy-hacky-nasty