def METHOD_NAME(cls, output_type):
    """Get the file extension for the given output type.
    Parameters
    ----------
    output_type : {'NIFTI', 'NIFTI_GZ', 'NIFTI_PAIR', 'NIFTI_PAIR_GZ'}
        String specifying the output type.
    Returns
    -------
    extension : str
        The file extension for the output type.
    """
    try:
        return cls.ftypes[output_type]
    except KeyError:
        msg = "Invalid FSLOUTPUTTYPE: ", output_type
        raise KeyError(msg)