mongoose/docs/c-api/http.h/mg_file_upload_handler.md

title	decl_name	symbol_kind	signature
mg_file_upload_handler()	mg_file_upload_handler	func	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. 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:

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;
  }
}