Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s07-multipart-reader.md

@@ -0,0 +1,71 @@
+---
+title: "S07. Receive Multipart Data as a Stream"
+order: 26
+status: "draft"
+---
+
+A naive upload handler puts the whole request into `req.body`, which blows up memory for large files. Use `HandlerWithContentReader` to receive the body chunk by chunk.
+
+## Basic usage
+
+```cpp
+svr.Post("/upload",
+  [](const httplib::Request &req, httplib::Response &res,
+     const httplib::ContentReader &content_reader) {
+    if (req.is_multipart_form_data()) {
+      content_reader(
+        // headers of each part
+        [&](const httplib::FormData &file) {
+          std::cout << "name: " << file.name
+                    << ", filename: " << file.filename << std::endl;
+          return true;
+        },
+        // body of each part (called multiple times)
+        [&](const char *data, size_t len) {
+          // write to disk here, for example
+          return true;
+        });
+    } else {
+      // plain request body
+      content_reader([&](const char *data, size_t len) {
+        return true;
+      });
+    }
+
+    res.set_content("ok", "text/plain");
+  });
+```
+
+The `content_reader` has two call shapes. For multipart data, pass two callbacks (one for headers, one for body). For plain bodies, pass just one.
+
+## Write directly to disk
+
+Here's how to stream an uploaded file to disk.
+
+```cpp
+svr.Post("/upload",
+  [](const httplib::Request &req, httplib::Response &res,
+     const httplib::ContentReader &content_reader) {
+    std::ofstream ofs;
+
+    content_reader(
+      [&](const httplib::FormData &file) {
+        if (!file.filename.empty()) {
+          ofs.open("uploads/" + file.filename, std::ios::binary);
+        }
+        return static_cast<bool>(ofs);
+      },
+      [&](const char *data, size_t len) {
+        ofs.write(data, len);
+        return static_cast<bool>(ofs);
+      });
+
+    res.set_content("uploaded", "text/plain");
+  });
+```
+
+Only a small chunk sits in memory at any moment, so gigabyte-scale files are no problem.
+
+> **Warning:** When you use `HandlerWithContentReader`, `req.body` stays **empty**. Handle the body yourself inside the callbacks.
+
+> For the client side of multipart uploads, see C07. Upload a File as Multipart Form Data.