def allowed_file(filename, allowed_set):
    """Checks if filename extension is one of the allowed filename extensions for upload.
    
    Args:
        filename: filename of the uploaded file to be checked.
        allowed_set: set containing the valid image file extensions.
    Returns:
        check: boolean value representing if the file extension is in the allowed extension list.
                True = file is allowed
                False = file not allowed.
    """
    check = '.' in filename and filename.rsplit('.', 1)[1].lower() in allowed_set
    return check