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client_max_body_size 50m; в таких случаях
в /etc/nginx/nginx.conf в секцию http

....

http {
    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;
    client_max_body_size 50m;
    
    ....