<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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]