JavaScript FormData() access file body

I have HTML form with file upload input. The form data are stored in a FormData() object so I can send it later to the server using AJAX.

What I’m trying to do is to access the FormData() values so I can modify the file content itself that appear in the request for example in Web Developer Tools -> Network.

If I’m uploading a .txt file or image, I want to be able to read its content, modify it and re-append it to the FormData().

My code so far is able to show the File object but I have no clue how to access its content.

for( var pair of myFormData.entries() ){

   /* pair[1] is the file object inside the FormData */
   console.log(pair[0], pair[1]);

}

Thank you.