File upload in a nested json object having multipart file in Angular 6

Question

Hi I am facing problem in uploading file in a object(body:DataSet) while submitting.

My classes are as below:

export class DataSet implements IDataSet {
Id?: number | undefined;
Name?: string | undefined;
isCorrect?: boolean | undefined;
data?: Data[] | undefined;
.
.
.
}
export class Data implements IData {
dataId?: number | undefined;
option?: string | undefined;
isDone?: boolean | undefined;
expectedFile: string|File|undefined;//file is to be sent here
.
.
.}

I want to send the file in this object -body.

I have tried using formdata but dont know how to append Data[] and the file in expectedFile along with Dataset. Also I tried using JSON.stringify but that produces empty object for Data.

Trying to upload it through angular in ui to server side java. Please guide with proper solution.