jellyfin/deployment/debian-package-x64/pkg-src/conf/logging.json

{
    "Serilog": {
        "MinimumLevel": "Information",
        "WriteTo": [
            {
                "Name": "Console",
                "Args": {
                    "outputTemplate": "[{Timestamp:HH:mm:ss}] [{Level:u3}] {Message:lj}{NewLine}{Exception}"
                }
            },
            {
                "Name": "Async",
                "Args": {
                    "configure": [
                        {
                            "Name": "File",
                            "Args": {
                                "path": "%JELLYFIN_LOG_DIR%//jellyfin.log",
                                "fileSizeLimitBytes": 10485700,
                                "rollOnFileSizeLimit": true,
                                "retainedFileCountLimit": 10,
                                "outputTemplate": "[{Timestamp:yyyy-MM-dd HH:mm:ss.fff zzz}] [{Level:u3}] {Message}{NewLine}{Exception}"
                            }
                        }
                    ]
                }
            }
        ]
    }
}