Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/http/upload.md

@@ -0,0 +1,36 @@
+---
+title: Handling file uploads
+---
+
+In order to handle file uploads, use the following HTML snippet:
+
+```HTML
+<form method="POST" action="/upload" enctype="multipart/form-data">
+  <input type="file" name="file">
+  <input type="submit" value="Upload">
+</form>
+```
+
+Uploaded files will be sent to the `/upload` endpoint via the `POST` request.
+HTTP body will contain multipart-encoded buffer with the file contents.
+
+To save the uploaded file, use this code snippet:
+
+```c
+struct mg_str cb(struct mg_connection *c, struct mg_str file_name) {
+  // Return the same filename. Do not actually do this except in test!
+  // fname is user-controlled and needs to be sanitized.
+  return file_name;
+}
+
+void ev_handler(struct mg_connection *c, int ev, void *ev_data) {
+  switch (ev) {
+    ...
+    case MG_EV_HTTP_PART_BEGIN:
+    case MG_EV_HTTP_PART_DATA:
+    case MG_EV_HTTP_PART_END:
+      mg_file_upload_handler(c, ev, ev_data, cb);
+      break;
+  }
+}
+```