how do I append readAsBinaryString or readAsDataURL to FormData

Question

I have this normal file upload, but what do I need is to send a file stored as a string in local storage along with FormData, because I cannot change anything on server. I will store the file locally using readAsBinaryString or readAsDataURL .

var reader = new FileReader();
/**
File reading code
reader.readAsBinaryString(file);    
**/
var fileString = reader.result

I need to send fileString into below formData.append instead of the imagefile.files[0]

Any idea?

var formData = new FormData();
var imagefile = document.querySelector('#file');
formData.append("image", imagefile.files[0]);
axios.post('upload_file', formData, {
    headers: {
      'Content-Type': 'multipart/form-data'
    }
})