def file_extensions_valid(ext_list):
    """Checks if file list contains only valid files

    Notes:
        Assumes all file extensions are equal! Only checks first file

    Args:
        ext_list (list): file extensions, eg ['.csv','.csv']

    Returns:
        bool: first element in list is one of ['.csv','.txt','.xls','.xlsx']?
    """
    ext_list_valid = ['.csv','.txt','.xls','.xlsx']
    return ext_list[0] in ext_list_valid