html:

<a name="downloadbtn" class="btn btn-success pull-right" href="/downloadfile/?filename=/root/allfile/123.txt">下载</a>

py:

@app.route('/downloadfile/', methods=['GET', 'POST'])
def downloadfile():
    if request.method == 'GET':
        fullfilename = request.args.get('filename')
　　　　# fullfilename = '/root/allfile/123.txt'
        fullfilenamelist = fullfilename.split('/')
        filename = fullfilenamelist[-1]
        filepath = fullfilename.replace('/%s'%filename, '')
        #普通下载
        # response = make_response(send_from_directory(filepath, filename, as_attachment=True))
        # response.headers["Content-Disposition"] = "attachment; filename={}".format(filepath.encode().decode('latin-1'))
        #return send_from_directory(filepath, filename, as_attachment=True)
        #流式读取
        def send_file():
            store_path = fullfilename
            with open(store_path, 'rb') as targetfile:
                while 1:
                    data = targetfile.read(20 * 1024 * 1024)   # 每次读取20M
                    if not data:
                        break
                    yield data

        response = Response(send_file(), content_type='application/octet-stream')
        response.headers["Content-disposition"] = 'attachment; filename=%s' % filename   # 如果不加上这行代码，导致下图的问题
        return response

没有文件名，和文件格式，遇到这种情况，打开F12，查看response.headers 与正常的比较