Erro 401 no upload

Acho que cometi algum erro, pois não estou conseguindo fazer o upload. Ao tentar chamar o método do upload recebo erro 401 Unauthorized do HTTP.

Segue abaixo o código:

--------------------
photo.service.ts

import { HttpClient, HttpParams } from "@angular/common/http";
import { Injectable } from "@angular/core";
import { Observable } from "rxjs";
import { Photo } from "./photo";

const API = 'http://localhost:3000';

@Injectable({providedIn: 'root'})
export class PhotoService
{
    constructor(protected http: HttpClient) {}

    public listFromUser(userName: string) : Observable<Photo[]>
    {
        const url = API + '/' +  userName.trim().toLowerCase() + '/photos';

        return this.http.get<Photo[]>(url);
    }

    public listFromUserPaginated(userName: string, page: number = 1) : Observable<Photo[]>
    {
        const url    = API + '/' +  userName.trim().toLowerCase() + '/photos';
        const params = new HttpParams().append('page', page.toString());

        return this.http.get<Photo[]>(url, { params });
    }

    public upload(description: string, allowComments: boolean, file: File) : Observable<Object>
    {
        const formData = new FormData();
        formData.append('description', description);
        formData.append('allowComments', allowComments ? 'true' : 'false');
        formData.append('imageFile', file);

        return this.http.post(API + '/photos/upload', formData);
    }
}

--------------------
photo-form.component.ts

import { Component, OnInit } from '@angular/core';
import { Router } from '@angular/router';
import { FormBuilder, FormGroup, Validators } from '@angular/forms';

import { PhotoService } from '../photo/photo.service';

@Component({
    selector   : 'ap-photo-form',
    templateUrl: './photo-form.component.html',
    styleUrls  : ['./photo-form.component.css']
})
export class PhotoFormComponent implements OnInit
{
    public photoForm: FormGroup;
    public file     : File;

    constructor(protected router: Router, protected formBuilder: FormBuilder, protected service: PhotoService) {}

    public ngOnInit() : void
    {
        this.photoForm = this.formBuilder.group({
            file         : ['', Validators.required],
            description  : ['', Validators.maxLength(300)],
            allowComments: [true],
        });
    }

    public upload() : void
    {
        const description   = this.photoForm.get('description').value;
        const allowComments = this.photoForm.get('allowComments').value;

        this.service
            .upload(description, allowComments, this.file)
            .subscribe(
                ()    => this.router.navigate(['']),
                error => console.log(error.message)
            );
    }
}

Aonde foi que eu errei?