Why can't I access req.body with multer?

My app (using vue) allows users to upload files with some info about the data to my node backend. When the user submits the form, this function is triggered:

methods: {
      buttonOK () {
        const formData = new FormData()

        formData.append('name', this.detailFirm.name)
        formData.append('description', this.detailFirm.description)
        formData.append('version', this.detailFirm.version)
        formData.append('date', this.detailFirm.date)
        formData.append('file', this.file)

        for (var [key, value] of formData.entries()) {
          console.log(key, value)
        }

        let headers = {
          'Content-Type': 'multipart/form-data',
          'Accept': 'multipart/form-data'
        }

        this.$http.put('/firmware', formData, {headers: headers})
        this.visible = false
      }

The log statement shows everything that it ought to, and when this request is made, the network tab in the chrome dev tools shows the post data going through, and it has all the values it should:

name: test
description: test
version: 1
date: 0555-05-05
file: (binary)

My multer middleware looks like this:

const multer = require('multer')
const mult = multer({
  dest: '/firmware'
})

module.exports = function (req, res, next) {
  /* --Convert multipart/form-data to useable format within express-- */
  if (req.path === '/firmware') {
    mult.single('file')
    console.log('MULTER MIDDLEWARE')
  }
  next()
}

The log statement there works, leading me to believe that multer is working.
I can't seem to access this information in back end though. Here I have tried both file and formData as the file name in mult.single('').

Here is my controller function:

let firmware = {
    name: req.body.name,
    version: req.body.version,
    description: req.body.description,
    date: req.body.date,
    file: req.body.file
  }
  firmwareRepo.create(firmware, (err, create) => {
                   .............

I've read some other questions, and have made a few adjustments, but I always get an empty object when I log req.body in the controller. Please advise.

I don't know why this worked, but everything started functioning as it should when I stopped using multer as an imported middleware, like this:

module.exports = function (req, res, next) {
  /* --Convert multipart/form-data to useable format within express-- */
  if (req.path === '/firmware') {
    mult.single('formData')
    console.log('MULTER MIDDLEWARE')
  }
  next()
}

and instead applied it directly to the route function, like this:

router.put('/firmware', upload.single('formData'), firmware.create) // doesn't work as standalone middleware

If anyone knows why that would be the case, please let me know.