以下是使用Python编写的解决方案代码示例：

import os

# 检查下载的文件是否存在于txt文件中
def detect_downloaded_files(filename, download_folder):
    downloaded_files = set()
    with open(filename, 'r') as f:
        for line in f:
            downloaded_files.add(line.strip())
    new_files = set(os.listdir(download_folder)) - downloaded_files
    for file in new_files:
        print("New file detected: ", file)

# 设置txt文件和下载文件夹的位置
filename = 'downloaded_files.txt'
download_folder = '/path/to/download/folder'

# 运行检测函数
detect_downloaded_files(filename, download_folder)

运行上面的代码将读取下载的文件列表文件和下载文件夹的位置，并提示任何新的下载文件。