我正在使用 react js 上传单个文件。但是 multer.single() 不起作用，而 multer.array() 起作用。问题是什么？

//Node.js
const upload = multer({
    storage: multer.diskStorage({
        destination: './uploads',
        filename: (_, file, cb) => cb(null, file.originalname),
    }),
});
// app.use(upload.array('pdf')); // works
app.use(upload.single('pdf')); // not working

使用单一方法时的错误是

(node:6096) UnhandledPromiseRejectionWarning: TypeError: Cannot read property '0' of undefined
    at convertToJPG (D:\Projects\Node\pdf2pic\controllers\convert.js:9:44)
    at Layer.handle [as handle_request] (D:\Projects\Node\pdf2pic\node_modules\express\lib\router\layer.js:95:5)
    at next (D:\Projects\Node\pdf2pic\node_modules\express\lib\router\route.js:137:13)
    at Route.dispatch (D:\Projects\Node\pdf2pic\node_modules\express\lib\router\route.js:112:3)
    at Layer.handle [as handle_request] (D:\Projects\Node\pdf2pic\node_modules\express\lib\router\layer.js:95:5)
    at D:\Projects\Node\pdf2pic\node_modules\express\lib\router\index.js:281:22
    at Function.process_params (D:\Projects\Node\pdf2pic\node_modules\express\lib\router\index.js:335:12)
    at next (D:\Projects\Node\pdf2pic\node_modules\express\lib\router\index.js:275:10)
    at Function.handle (D:\Projects\Node\pdf2pic\node_modules\express\lib\router\index.js:174:3)
    at router (D:\Projects\Node\pdf2pic\node_modules\express\lib\router\index.js:47:12)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:6096) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:6096) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

客户端代码是

<input
          accept=".pdf"
          multiple={false}
          name="pdf"
          onChange={onChangeHandler}
          ref={fileInputRef}
          style={{ display: 'none' }}
          type="file"
        />

    const onChangeHandler = (event: React.ChangeEvent<HTMLInputElement>) => {
      if (!event.target.files?.length) {
        return;
      }
  
      const formData = new FormData();  
      Array.from(event.target.files).forEach((file) => {
        formData.append(event.target.name, file);
      });
  
      props.onChange(formData);
  
      formRef.current?.reset();
    };

我在这里做错了什么？