Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s06-download-response.md

@@ -0,0 +1,50 @@
+---
+title: "S06. Return a File Download Response"
+order: 25
+status: "draft"
+---
+
+To force a browser to show a **download dialog** instead of rendering inline, send a `Content-Disposition` header. There's no special cpp-httplib API for this — it's just a header.
+
+## Basic usage
+
+```cpp
+svr.Get("/download/report", [](const httplib::Request &req, httplib::Response &res) {
+  res.set_header("Content-Disposition", "attachment; filename=\"report.pdf\"");
+  res.set_file_content("reports/2026-04.pdf", "application/pdf");
+});
+```
+
+`Content-Disposition: attachment` makes the browser pop up a "Save As" dialog. The `filename=` parameter becomes the default save name.
+
+## Non-ASCII file names
+
+For file names with non-ASCII characters or spaces, use the RFC 5987 `filename*` form.
+
+```cpp
+svr.Get("/download/report", [](const httplib::Request &req, httplib::Response &res) {
+  res.set_header(
+    "Content-Disposition",
+    "attachment; filename=\"report.pdf\"; "
+    "filename*=UTF-8''%E3%83%AC%E3%83%9D%E3%83%BC%E3%83%88.pdf");
+  res.set_file_content("reports/2026-04.pdf", "application/pdf");
+});
+```
+
+The part after `filename*=UTF-8''` is URL-encoded UTF-8. Keep the ASCII `filename=` too, as a fallback for older browsers.
+
+## Download dynamically generated data
+
+You don't need a real file — you can serve a generated string as a download directly.
+
+```cpp
+svr.Get("/export.csv", [](const httplib::Request &req, httplib::Response &res) {
+  std::string csv = build_csv();
+  res.set_header("Content-Disposition", "attachment; filename=\"export.csv\"");
+  res.set_content(csv, "text/csv");
+});
+```
+
+This is the classic pattern for CSV exports.
+
+> **Note:** Some browsers will trigger a download based on Content-Type alone, even without `Content-Disposition`. Conversely, setting `inline` tries to render the content in the browser when possible.