Convert upload file to base64

[KhaiNguyen01]

I want to convert the upload file to base64, is there any way to do that?
I have tried to catch action 'sending' like this
*.component.html:
<dropzone [config]="config" (error)="onUploadError($event)" (success)="onUploadSuccess($event)" (sending)="onSending($event)">
*.component.ts:
onSending(event) {
const reader = new FileReader();
var self = this;
event[2].append({'custom_file': '12334'});
}
But the processing continue before the customfile was appended into event[2]