如何将文件从angular 10上载到asp.net core WebAPI 3.1

我正在尝试将Angular 10应用程序中的文件（图像或PDF）上载到dotNET Core 3.1 web API。 但是没有成功。下面是我的代码

WebAPI代码 当我点击上传。出现以下错误

有人能帮忙吗？实际上，此代码在dotNET Core 2.2中有效，但在3.1中无效。

在.net Core 3中使用接口

    [HttpPost]
    [Route("UploadDocument")]
    public async Task<IActionResult> UploadDocument([FromForm] IFormFile avatar)
    {
        using (var memoryStream = new MemoryStream())
        {
            await avatar.CopyToAsync(memoryStream);
            // Upload the file if less than 2 MB
            if (memoryStream.Length > 2097152)
            {
                ModelState.AddModelError("File", "The file is too large.");
                return BadRequest(ModelState);
            }
            return Created("","");
        }
    }

[HttpPost]
[路线（“上传文件”）]
公共异步任务上载文档（[FromForm]IFormFile avatar）
{
使用（var memoryStream=new memoryStream（））
{
等待化身。复制同步（memoryStream）；
//如果小于2 MB，请上载文件
如果（memoryStream.Length>2097152）
{
AddModelError（“文件”，“文件太大”）；
返回请求（ModelState）；
}
已创建的返回（“，”）；
}
}

这可能会有所帮助。

 <form [formGroup]="caseForm" (ngSubmit)="saveCase()">
        <div class="row">
            <div class="col-md-3">
                Application Name
            </div>
            <div class="col-md-6">
                <input type="text" class="form-control" formControlName="ApplicantName">
            </div>
        </div>

        <div class="row mt-3">
            <div class="col-md-3">
                Select Document
            </div>
            <div class="col-md-6">
                <input type="file" class="form-control" (change)="onFileSelected($event)">
                <br>
                <span [innerHTML]="result"></span>
            </div>
        </div>

        <div class="row mt-3">
            <div class="col-md-3">

            </div>
            <div class="col-md-3">
                <button type="button" class="btn btn-block btn-secondary">Clear</button>
            </div>
            <div class="col-md-3">
                <button type="submit" class="btn btn-block btn-primary">Save</button>
            </div>
        </div>
    </form>

 saveCase() {
    if (this.caseForm.valid) {
      const formData = new FormData();

      formData.append("ApplicantName", this.caseForm.value.ApplicantName);
      formData.append("file", this.selectedFile, this.selectedFile.name);

      // Display the values
      formData.forEach((value, key) => {
        console.log(key + value)
      });

      this.http.post("https://localhost:44343/api/Case/UploadDocument", formData, {
        reportProgress: true,
        observe: 'events'
      })
        .subscribe(event => {
          if (event.type === HttpEventType.UploadProgress) {
            console.log("Upload Progress: " + Math.round(event.loaded / event.total * 100) + '%')
          }
          else if (event.type === HttpEventType.Response) {
            console.log(event);
            this.toastr.success('Case', 'Case Created Successfully');
          }

          this.resetCaseForm();
        },
          err => {
            console.error(err);
          });
    }
  }

    [HttpPost]
    [Route("UploadDocument")]
    public async Task<IActionResult> UploadDocument([FromForm] IFormFile avatar)
    {
        using (var memoryStream = new MemoryStream())
        {
            await avatar.CopyToAsync(memoryStream);
            // Upload the file if less than 2 MB
            if (memoryStream.Length > 2097152)
            {
                ModelState.AddModelError("File", "The file is too large.");
                return BadRequest(ModelState);
            }
            return Created("","");
        }
    }