...
Content-Type: multipart/form-data; boundary=${boundary} 

--${boundary}
...
...

--${boundary}--

POST http://www.example.com HTTP/1.1
Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryyb1zYhTI38xpQxBK

------WebKitFormBoundaryyb1zYhTI38xpQxBK
Content-Disposition: form-data; name="city_id"

1

------WebKitFormBoundaryyb1zYhTI38xpQxBK
Content-Disposition: form-data; name="company_id"

2
------WebKitFormBoundaryyb1zYhTI38xpQxBK
Content-Disposition: form-data; name="file"; filename="chrome.png"
Content-Type: image/png

PNG ... content of chrome.png ...
------WebKitFormBoundaryyb1zYhTI38xpQxBK--

var formData = new FormData();
formData.append("username", "Groucho");
formData.append("accountnum", 123456); 
fetch('/users', {
 method: 'POST',
 body: formData
})

------WebKitFormBoundaryyb1zYhTI38xpQxBK
Content-Disposition: form-data; name="user"

[object Object]

var obj = {
  a: '2', 
  b: {c: 'test'}, 
  c: [ 
    {id: 1, name: 'xx'}, 
    {id:2 ,name: 'yy', info: {d: 4} }
  ]
}

a: 2
b[c]: test
c[][id]: 1
c[][name]: xx
c[][id]: 2
c[][name]: yy
c[][info][d]:4

function objectToFormData (obj, form, namespace) {
 const fd = form || new FormData();
 let formKey;
 
 for(var property in obj) {
   if(obj.hasOwnProperty(property)) {
    let key = Array.isArray(obj) ? '[]' : `[${property}]`;
    if(namespace) {
     formKey = namespace + key;
    } else {
     formKey = property;
    }
   
    // if the property is an object, but not a File, use recursivity.
    if(typeof obj[property] === 'object' && !(obj[property] instanceof File)) {
     objectToFormData(obj[property], fd, formKey);
    } else {
     
     // if it's a string or a File object
     fd.append(formKey, obj[property]);
    }
    
   }
  }
 
 return fd;
  
}