Purpose of Content-Type undefined in AngularJS FileUpload

Question

For uploading files from angular to a Spring MVC REST WebService, people seem to be setting Content-Type : undefined header in the AJAX request.

What is the purpose of this type of content-type and is it required by Spring MultiPart?

$http.post(uploadUrl, fd, {
  transformRequest : angular.identity,
  headers : {
   'Content-Type' : undefined
  }
//...
}

Answer 1

It will reset the default header 'application/json' and let the browser fill in the correct Content-Type for us, along with a correct boundary parameter.

Setting in manually to 'multipart/form-data', for example, will fail to add the already mentioned boundary.