Javascript 附加的 FormData 将 null 值发送到 API

在我的Angular项目中，我在上传之前使用了ngx-image-cropper来裁剪图像.component.ts的一些相关部分包括:

croppedImage: any = '';
imageCropped(event: ImageCroppedEvent) {
  this.croppedImage = event.base64;
}
Base64ToFile(url: any, filename: any, mimeType: any){
    return (fetch(url)
            .then(function(res){return res.arrayBuffer();})
            .then(function(buf){return new File([buf], filename,{type:mimeType});})
        );
  }

 OnSubmit() {
    const filedata = new FormData();
    this.Base64ToFile(this.croppedImage, "myphoto.png", "image/png").then(function(outputFile){
      
      console.log(outputFile);
      filedata.append("file", outputFile, outputFile.name);//The first parameter "file" is the input parameter in API method
    });
    this.http.post('https://localhost:5001/api/featuredpost/postimage/' + this.selectedPostId, filedata).subscribe(
      response => {
        console.log(response);
      }
    );
  }

API action:

public async Task<IActionResult> PostImage([FromForm] IFormFile file, [FromRoute] int id){
    ...
}

问题是，当我调试API时，input file参数为null，而cropped image的字符串值为base64.我该怎么修？