Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/c-api/http_server.h/mg_file_upload_handler.md

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+---
+title: "mg_file_upload_handler()"
+decl_name: "mg_file_upload_handler"
+symbol_kind: "func"
+signature: |
+  void mg_file_upload_handler(struct mg_connection *nc, int ev, void *ev_data,
+                              mg_fu_fname_fn local_name_fn);
+---
+
+File upload handler.
+This handler can be used to implement file uploads with minimum code.
+This handler will process MG_EV_HTTP_PART_* events and store file data into
+a local file.
+`local_name_fn` will be invoked with whatever name was provided by the client
+and will expect the name of the local file to open. A return value of NULL
+will abort file upload (client will get a "403 Forbidden" response). If
+non-null, the returned string must be heap-allocated and will be freed by
+the caller.
+Exception: it is ok to return the same string verbatim.
+
+Example:
+
+```c
+struct mg_str upload_fname(struct mg_connection *nc, struct mg_str fname) {
+  // Just return the same filename. Do not actually do this except in test!
+  // fname is user-controlled and needs to be sanitized.
+  return fname;
+}
+void ev_handler(struct mg_connection *nc, 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(nc, ev, ev_data, upload_fname);
+      break;
+  }
+}
+``` 
+