docs: 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]