flask vs Nginx upload, nginx setting

upload

    upstream frontends {
        server 127.0.0.1:8222;
    }

    server {
        listen 8666;

        # Allow file uploads max 50M for example
        client_max_body_size 50M;
	    upload_buffer_size 16M;

        # POST URL
        location /upload {
            # Pass altered request body to this location
            upload_pass @after_upload;

            # Store files to this directory
            upload_store /tmp/xyz/;

            # Allow uploaded files to be world readable
            upload_store_access user:rw group:rw all:r;

            # Set specified fields in request body
            upload_set_form_field $upload_field_name.name "$upload_file_name";
            upload_set_form_field $upload_field_name.content_type "$upload_content_type";
            upload_set_form_field $upload_field_name.path "$upload_tmp_path";

            # Inform backend about hash and size of a file
            upload_aggregate_form_field $upload_field_name.md5 "$upload_file_md5";
            upload_aggregate_form_field $upload_field_name.size "$upload_file_size";

            upload_pass_form_field "^newfilename$|^market$";

            upload_cleanup 400 404 499 500-505;
        }

        location @after_upload {
            proxy_pass   http://frontends;
        }

        location / {
            proxy_pass_header Server;
            proxy_set_header Host $http_host;
            proxy_redirect off;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Scheme $scheme;
            proxy_pass http://frontends;
        }
    }