Serving images with mongoose in C++ causes headers to corrupt data

Mongoose 7.8, C++ Standard 17

When uploading files with mg_http_serve_file the associated headers get baked into the file before the actual file headers, leading them to all be unreadable.

As an example, a .jpg uploaded would look like:

HTTP/1.1 304 Not Modified
Content-Length: 0000000000

����...JFIF // The actual .jpg data

Following guides, this is my event handler:

namespace fs = std::filesystem

static void evt_handler(struct mg_connection* c, int ev, void* ev_data, void* fn_data)
{
  if (ev == MG_EV_HTTP_MSG)
  {
    struct mg_http_message* hm = (struct mg_http_message*)ev_data;
    struct mg_http_serve_opts opts = {};

    std::string s = hm->uri.ptr;

    std::string buf = "";
    int i = 0;
    while (s[i] != ' ') { buf += s[i++]; }

    // If requesting image
    if (buf.find("/img/") != std::string::npos)
    {
      i = 5;
      buf = "";
      while (s[i] != ' ') { buf += s[i++]; }

      fs::path p = fs::current_path();
      fs::path z = "\\html\\img\\" + buf;
      p += z;

      mg_http_serve_file(c, hm, p.string().c_str(), &opts);
    }
    mg_http_serve_file(c, hm, "./html/index.html", &opts);
  }
}

Initialization

int initServer(const char* url)
{
  struct mg_mgr mongoose_manager;
  mg_mgr_init(&mongoose_manager);
  mg_http_listen(&mongoose_manager, url, evt_handler, NULL);
  for (;;)
  {
    mg_mgr_poll(&mongoose_manager, 1000);
  }
  mg_mgr_free(&mongoose_manager);
  return 0;
};

Main

int main(int argc, char const* argv[])
{
  const char* url = "127.0.0.1:9080";
  initServer(url);
  return 0;
}

The files appear to still be valid, but the garbage data is added somewhere along the way.