def METHOD_NAME(mime_type_or_extension: str) -> str:
    extensions = {
        '.csv': ',',
        '.tsv': '\t',
        '.txt': '\t',
        '.tab': '\t',
        'text/csv': ',',
        'text/tab-separated-values': '\t',
    }
    default_separator = ','
    return extensions.get(mime_type_or_extension.lower(), default_separator)