Add Cookbook C01-C19 (draft)

docs-src/pages/en/cookbook/c07-multipart-upload.md

@@ -0,0 +1,52 @@
+---
+title: "C07. Upload a File as Multipart Form Data"
+order: 7
+status: "draft"
+---
+
+When you want to send a file the same way an HTML `<input type="file">` does, use multipart form data (`multipart/form-data`). cpp-httplib offers two APIs — `UploadFormDataItems` and `FormDataProviderItems` — and you pick between them based on **file size**.
+
+## Send a small file
+
+Read the file into memory first, then send it. For small files, this is the simplest path.
+
+```cpp
+httplib::Client cli("http://localhost:8080");
+
+std::ifstream ifs("avatar.png", std::ios::binary);
+std::string content((std::istreambuf_iterator<char>(ifs)),
+                     std::istreambuf_iterator<char>());
+
+httplib::UploadFormDataItems items = {
+  {"name", "Alice", "", ""},
+  {"avatar", content, "avatar.png", "image/png"},
+};
+
+auto res = cli.Post("/upload", items);
+```
+
+Each `UploadFormData` entry is `{name, content, filename, content_type}`. For plain text fields, leave `filename` and `content_type` empty.
+
+## Stream a large file
+
+To avoid loading the whole file into memory, use `make_file_provider()`. It reads the file in chunks as it sends — so even huge files won't blow up your memory footprint.
+
+```cpp
+httplib::Client cli("http://localhost:8080");
+
+httplib::UploadFormDataItems items = {
+  {"name", "Alice", "", ""},
+};
+
+httplib::FormDataProviderItems provider_items = {
+  httplib::make_file_provider("video", "large-video.mp4", "", "video/mp4"),
+};
+
+auto res = cli.Post("/upload", httplib::Headers{}, items, provider_items);
+```
+
+The arguments to `make_file_provider()` are `(form name, file path, file name, content type)`. Leave the file name empty to use the file path as-is.
+
+> **Note:** You can mix `UploadFormDataItems` and `FormDataProviderItems` in the same request. A clean split is: text fields in `UploadFormDataItems`, files in `FormDataProviderItems`.
+
+> To show upload progress, see C11. Use the progress callback.