angular 8 表单带文件上传接口

<div >

    <form (ngSubmit)="doSubmit()" enctype="multipart/form-data">

                    <mat-label>作业标题</mat-label>
                    <mat-icon matSuffix class="secondary-text">account_circle</mat-icon>
                    <input name="title" required [(ngModel)]="homeWork.title">
        <br>

                    <mat-label>作业内容</mat-label>
                    <textarea name="description" [(ngModel)]="homeWork.description" max-rows="4"></textarea>
        <br>
                    <mat-label>文件</mat-label>
                    <input name="file" type="file" (change)="upload($event)">
        <br>

        <button *ngIf="homeWork.id !== ''"
                aria-label="Delete"
                matTooltip="Delete">
            DELETE
        </button>

        <input type="submit" aria-label="SAVE" value="SAVE">

    </form>

</div>

    upload(e): void{
        this.file = e.target.files[0];
    }

    doSubmit(): void {
        const formData: FormData = new FormData();

        formData.append('file', this.file);
        formData.append('id', this.homeWork.id);
        formData.append('title', this.homeWork.title);
        formData.append('description', this.homeWork.description);
        formData.append('teacher_id', this.authentication.id);
        formData.append('course_id', this.courseId);

        this._courseService.updateHomeWork(formData).then( (res) => {
            alert(res.msg);
        } );


    }

    /**
     * 保存或者更新老师布置的作业
     * @param formData ，老师布置的作业
     */
    updateHomeWork(formData ): Promise<any>
    {
        return new Promise((resolve, reject) => {
            const header: HttpHeaders = new HttpHeaders();
            header.set('Content-Type', 'multipart/form-data');
            this._httpClient.post(this._fuseConfigService.configSnapshot.url + '/api.php/rest/homeworks/file',
                formData,
                {headers: header }
                )
                .subscribe(response => {
                    // 添加成功后重新获取最新的作业信息
                    this.getHomeWork();
                    resolve(response);
                });

        });
    }