如果上传文件目录不在项目的public中，则对项目的上传目录进行公开，才能让外界访问到上传的文件。这需要对web服务器设置下
我这里使用的是nginx
nginx设置如下：

#设置后台
        server {
    
    
        charset utf-8;
        client_max_body_size  50m;

        listen 9000; ## listen for ipv4
        #listen [::]:80 default_server ipv6only=on; ## listen for ipv6

        server_name www.frontend.com;
        root        E:/2020PHPStudy/phpstudy_pro/WWW/customer-php/backend/web/;
        index       index.php;

        location / {
    
    
            # Redirect everything that isn't a real file to index.php
            try_files $uri $uri/ /index.php$is_args$args;
        }

        # uncomment to avoid processing of calls to non-existing static files by Yii
        #location ~ \.(js|css|png|jpg|gif|swf|ico|pdf|mov|fla|zip|rar)$ {
    
    
        #    try_files $uri =404;
        #}
        #error_page 404 /404.html;

        # deny accessing php files for the /assets directory
        location ~ ^/assets/.*\.php$ {
    
    
            deny all;
        }

        ...
		
		
        #此处为上传文件目录公开设置
        location /uploads {
    
    
            alias E:\2020PHPStudy\phpstudy_pro\WWW\customer-php\uploads;
            access_log  off;
            expires  360d;
            try_files  $uri =403;
        }
}