def METHOD_NAME(self, data):
    while None in data:  # Got data == [None] when there is no files.
        data.remove(None)
    super(MultiFileField, self).METHOD_NAME(data)
    num_files = len(data)
    if len(data) and not data[0]:
        num_files = 0
    if num_files < self.min_num:
        raise ValidationError(self.error_messages['min_num'] % {'min_num': self.min_num, 'num_files': num_files})
        return
    elif self.max_num and num_files > self.max_num:
        raise ValidationError(self.error_messages['max_num'] % {'max_num': self.max_num, 'num_files': num_files})
    for uploaded_file in data:
        if uploaded_file.size > self.maximum_file_size:
            raise ValidationError(self.error_messages['file_size'] % {'uploaded_file_name': uploaded_file.name})