How can i check the form data sent using multipart/form-data header before passing it to multer middleware

I have FormData object sent from react app. It contains email and audio data with multipart/form-data header, i want to check the req.body in the first middleware checkUser before passing it to multer middleware audioUpload (before uploading the audio).

FormData

    const handleSendAudio = async () => {
        let fd = new FormData();
        fd.append("email", email);
        fd.append("audio", audioData);

        try {
            const resp = await authFetch.post("/complaints/audio", fd, {
                headers: { "Content-Type": "multipart/form-data" },
            });

        } catch (ex) {
           console.log(ex);
         }
    };

API side: file: checkUser.js

module.exports = async function (req, res, next) {
    
    const user = await User.findOne({ email: req.body.email });
    if (!user)
        return res.status(401).json({ error: "cannot perform such operation now" });

    next();
};

file: route.js

  router.post(
    "/audio",
     checkUser, // i want to check the body here before passing it to multer middleware but the req.body is empty here.
    audioUpload.single("audio"), //multer middleware
    async (req, res) => {
        
        const audioPath = req.file.path;
        const {  email } = req.body;

        console.log("audioPath: ", audioPath);

        const complaint = new Complaint({
            email,
            audioUrl: audioPath,
        });

        await complaint.save();

        res.status(200).json(complaint);
    }
);