def METHOD_NAME(self):
    cleaned_data = self.cleaned_data
    for name, field in self.fields.items():
        if isinstance(field, FileField):
            file_data = cleaned_data[name]
            if file_data:
                file_name = file_data.name
                file_extension = file_name.split('.')[-1]
                if file_extension not in settings.FILE_ALLOWED_EXTENSIONS:
                    self.add_error(name, f"File extension '{file_extension}' is not allowed. "
                                         f"Allowed extensions are: '{', '.join(settings.FILE_ALLOWED_EXTENSIONS)}'")
    return self.cleaned_data