如何在angular 4中对<input type="file">使用所需的验证

import { FormsModule, ReactiveFormsModule } from '@angular/forms';

<form [formGroup]="myForm">
  <input type="file" formControlName="myFile">
  <div *ngIf="myForm.get('myFile').invalid && (myForm.get('myFile').dirty || myForm.get('myFile').touched)">
    <div *ngIf="myForm.get('myFile').errors.required">请选择一个文件</div>
    <div *ngIf="myForm.get('myFile').errors.maxSize">文件大小超过限制</div>
    <div *ngIf="myForm.get('myFile').errors.fileType">文件类型不支持</div>
  </div>
</form>

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

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponent {
  myForm: FormGroup;

  constructor(private formBuilder: FormBuilder) {
    this.myForm = this.formBuilder.group({
      myFile: ['', [Validators.required, this.fileSizeValidator, this.fileTypeValidator]]
    });
  }

  fileSizeValidator(control: FormControl): { [key: string]: any } {
    const maxSize = 1024; // 限制文件大小为1MB
    const fileSize = control.value.size / 1024; // 将文件大小转换为KB
    return fileSize > maxSize ? { 'maxSize': true } : null;
  }

  fileTypeValidator(control: FormControl): { [key: string]: any } {
    const allowedTypes = ['image/jpeg', 'image/png']; // 限制文件类型为JPEG和PNG
    return allowedTypes.includes(control.value.type) ? null : { 'fileType': true };
  }
}