Node.js 使用 nodeexpressMulter，您如何首先判断参数

我就是这样用穆特的，

const specialExample = multer({ .. blah blah })
app.post(
    '/specialPath',
    specialExample.single('handled_by_multer'),
    await function (req, res) {
        fs.renameSync(.. move the file, etc
        // send a reply
        res.set('Content-Type', 'text/plain')
        res.status(200).send(`ok`)
    })

所以请注意，事情按如下顺序发生:

app.post(
    '/specialPath',
// EXPRESS KNOWS TO GO THIS WAY
    specialExample.single('handled_by_multer'),
// MULTER LITERALLY IS GETTING THE FILE
// MULTER HAS COMPLETELY GOT AND SAVED THE FILE
    await function (req, res) {
// THIS FUNCTION NOW BEGINS RUNNING
        ... etc

这一切都很好.

现在，想象一下，我想看看一些参数.

在某些情况下，我DO NOT想要得到文件.

const specialExample = multer({ .. blah blah })
app.post(
    '/specialPath',
    specialExample.single('handled_by_multer'),
    await function (req, res) {
        const meta = req.body.meta
        if (meta !== "wth") {
           .. we actually DON'T want the  file
           .. delete the file
           .. send a 401 and return
        }
        fs.renameSync(.. move the file, etc
        // send a reply
        res.set('Content-Type', 'text/plain')
        res.status(200).send(`ok`)
    })

请注意，这可以很好地工作，但是，我们正在浪费地获取/保存文件(然后才删除它).

有没有办法做更多这样的事情......

app.post(
    '/specialPath',

    await function (req, res) {
        .. if I happen to want to, only then: {
            specialExample.single('handled_by_multer'),
        }
    })

怎么做？