JavaScript Object to FormData, with support for nested objects, arrays and File objects. Includes Angular.js usage.

object-to-form-data.js

// takes a {} object and returns a FormData object
var objectToFormData = function(obj, form, namespace) {
    
  var fd = form || new FormData();
  var formKey;
  
  for(var property in obj) {
    if(obj.hasOwnProperty(property)) {
      
      if(namespace) {
        formKey = namespace + '[' + property + ']';
      } 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, property);
        
      } else {
        
        // if it's a string or a File object
        fd.append(formKey, obj[property]);
      }
      
    }
  }
  
  return fd;
    
};
  
// usage example
var z = objectToFormData({
    obj: {
      prop: 'property value'
    },
    arr: [
      'one',
      'two',
      'three',
      new File([''], '')
    ],
    file: new File([''], '')
});

var xhr = new XMLHttpRequest;
xhr.open('POST', '/', true);
xhr.send(z);
  
  
// usage for Angular.js

// wrap object to formdata method,
// to use it as a transform with angular's http.
var formDataTransform = function(data, headersGetter) {
    
  // we need to set Content-Type to undefined,
  // to make the browser set it to multipart/form-data 
  // and fill in the correct *boundary*.
  // setting Content-Type to multipart/form-data manually 
  // will fail to fill in the boundary parameter of the request.
  headersGetter()['Content-Type'] = undefined;
    
  return objectToFormData(data);

};
  
 $http({
  method: 'POST',
  url: '/',
  transformRequest: formDataTransform,
  data: { your_object: {} }
})
.success(function(res) {});

Namespace is lost. This could fix it:
objectToFormData(obj[property], fd, namespace+"["+ property+"]");

Update for Typescript 2022.
Edit: Date support removed.

function parseGenericObject<T>(
   object: T,
   form?: FormData,
   namespace?: string
 ): FormData {
   const formData = form || new FormData()
   for (const property in object) {
     const isPropertyExist = property in object

     if (!isPropertyExist) {
       continue
     }

     const contextProperty = object[property]
     const formKey = namespace ? `${namespace}[${property}]` : property

     if (
       typeof contextProperty === 'object' &&
       !(contextProperty instanceof File)
     ) {
       parseGenericObject<any>(contextProperty, formData, formKey)
     } else {
       formData.append(formKey, String(contextProperty))
     }
   }
   return formData
 }

This does not work. It obviously is not nesting anything.

The server receives: {'p1[p1]' (all one key): val} instead of {'p1': {'p2': val}}