POST file along with form data Vue + axios

I have a method for Vuejs component:

async submit () {
        if (this.$refs.form.validate()) {
          let formData = new FormData()
          formData.append('userImage', this.avatarFile, this.avatarFile.name)
          this.avatarFile = formData
          try {
            let response = await this.$axios.post('http://localhost:3003/api/test.php', {
              avatar: this.avatarFile,
              name: this.name,
              gender: this.gender,
              dob: this.DOB,
            }, {
              headers: {
                'Content-Type': 'multipart/form-data; boundary=' + formData._boundary
              }
            })
            if (response.status === 200 && response.data.status === 'success') {
              console.log(this.response)
            }
          } catch (e) {
           console.log(e)
          }
        }
      }

And in test.php, I'm using json_decode(file_get_contents("php://input"), TRUE); to read data as $_POST variables.

While I am able to read name, gender and dob correctly, I can't fetch avatar properly.

Any solutions for the same?

Note: I don't to append every variable as formData.append(.., ..) as I am planning to handle over 14 variables.

Note for moderators: I didn't find any question where formData was being used along with other data objects.

So, I figured this one out in a simpler way:

    let rawData = {
                name: this.name,
                gender: this.gender,
                dob: this.dob
              }
              rawData = JSON.stringify(rawData)
    let formData = new FormData()
          formData.append('avatar', this.avatarFile, this.avatarFile.name)
          formData.append('data', rawData)
    try {
            let response = await this.$axios.post('http://localhost:3003/api/test.php', formData, {
              headers: {
                'Content-Type': 'multipart/form-data'
              }
         })

test.php:

$_POST = json_decode($_POST['data'],true);

Note: I had an option of using:

Object.keys(rawData).map(e => {
            formData.append(e, rawData[e])
          })

but since I was dealing with nested objects (name: { first: '', last: ''} ) in rawData, I chose not to do that as it would require recursive methods on either client side or server side.