def find_by_fileext(fileext, prs):
    """
    :param fileext: File extension
    :param prs: A list of :class:`anyconfig.models.processor.Processor` classes
    :return: A list of processor class to processor files with given extension
    :raises: UnknownFileTypeError
    """
    def pred(pcls):
        """Predicate"""
        return fileext in pcls.extensions()

    pclss = findall_with_pred(pred, prs)
    if not pclss:
        raise UnknownFileTypeError("file extension={}".format(fileext))

    return pclss