Form data is empty after using append?

I have a problem to append anyhing in form data. Its always empty. How can i append image file and some other data ?

  var data = new FormData();
    fileResult = this.$els.fileIntroImage.files;

    data.append('name',this.property_credentials.name);
                        data.append('default_image',this.$els.fileIntroImage.files[0],File);
    console.log('data',data);

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