[Vuejs]-How to send mail with multiple file attachements

You add content to FormData with the append method.

So your code should be something like this:

onFileChanged (e) {
  this.form = new FormData(); // maybe it's better to initialize your form outside of this function
  for (const file of e.target.files) {
    if (file.type) {
      this.form.append('files[]', file)
    }
  }
},

See also this answer.