Reactjs 为什么可以'；部署MERN堆栈项目时是否上载映像？

在本地连接到服务器并尝试上载映像时，效果良好，但在heroku上部署项目并尝试上载映像时，他会发送以下错误：

2021-04-11T05:45:01.411678+00:00 app[web.1]: [Error: ENOENT: no such file or directory, open './public/img/photo_1618119901407.jpg'] {
2021-04-11T05:45:01.411680+00:00 app[web.1]:   errno: -2,
2021-04-11T05:45:01.411680+00:00 app[web.1]:   code: 'ENOENT',
2021-04-11T05:45:01.411681+00:00 app[web.1]:   syscall: 'open',
2021-04-11T05:45:01.411681+00:00 app[web.1]:   path: './public/img/photo_1618119901407.jpg'
2021-04-11T05:45:01.411682+00:00 app[web.1]: }
2021-04-11T05:45:01.411986+00:00 app[web.1]: 500 

controller/post.js：

let file = req.files.image;
file.name = `photo_${Date.now()}${path.parse(file.name).ext}`;
file.mv(`${process.env.FILE_UPLOAD_PATH}${file.name}`, async err => {

    if (err) {
        console.log(err)
        return next(500, `problem with file upload`)
    }

    // save new post in db
    req.body.imageUrl = file.name;
    result = await postSchema.create({
        title: req.body.title,
        content: req.body.content,
        imageUrl: req.body.imageUrl,
        creator: {
            id: req.user.id,
            name: req.user.name
        },
    })
})