def defaultExtension(self):
        """Return default extension for this parameter type, checked
        against supported fileExtensions.  If the default extension is
        not within `fileExtensions`, return the first supported
        extension."""
        result = self.EXTERNAL_TYPES[self.typ]
        if not self.fileExtensions:
            return result
        if result in self.fileExtensions:
            return result
        return self.fileExtensions[0]