Answer
This isn't achievable natively — the underlying interface for Quick Report email output (ExcelQuickReport) always produces an ANSI-encoded Excel file (.xlsx), regardless of what file extension you configure for the output. Simply changing the configured output extension to .csv doesn't actually convert the file's format or encoding — it just renames the file, and Excel will show a format-mismatch warning when opening it (though it will still open after confirming). There's no built-in way around this; the only real solution is a manual conversion step after delivery — opening the file in a client tool like Excel and using Save As to explicitly save it as a genuine UTF-8 file.