docs: def clean_key_name(self):
          """Validates that the key in the provided data starts with the
          required prefix, and that it exists in the bucket."""
          key = self.cleaned_data['key_name']
          # Ensure key starts with prefix
          if not key.startswith(self.get_key_prefix()):
              raise forms.ValidationError('Key does not have required prefix.')
          # Ensure key exists
          if not self.get_upload_key():
              raise forms.ValidationError('Key does not exist.')
          return key