<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def invalid_content_type(self, request=None, response=None):
 """Returns the content type that should be used by default on validation errors"""

        if callable(self.invalid_outputs.content_type):
            return self.invalid_outputs.content_type(request=request, response=response)
        else:
            return self.invalid_outputs.content_type