问为什么上传文件时react.js会返回cors

EN

const formData = new FormData();
    formData.append("CustomerName", this.state.customerName);
    formData.append("Email", this.state.email);
    formData.append("Phone", this.state.phone);
    formData.append("PageNumber", this.state.pagesNumber);
    formData.append("Notes", this.state.notes);
    formData.append("WritingConversionTypeId", this.state.writingConversionTypeId);
    formData.append("WritingDocumentTypeId", this.state.writingDocumentTypeId);
    formData.append("WritingTimePeriodId", this.state.writingTimePeriodId);
    formData.append("files", 'null');
    
     writingRequest.postwritingRequest(formData).then((res) => {
      console.log(res);
    });

at React Service to Call Api

import http from "../../config/http";
import endPoints from "../endPoints";

const writingRequestUrl = endPoints.WRITING_REQUEST_ENDPOINT;

export default {
  postwritingRequest(writingRequest) {
    return http
      .post(
        writingRequestUrl,
        writingRequest
          , {
          headers: {
            'enctype': 'multipart/form-data',
            "Content-Type": "multipart/form-data"
          },
        }
      )
      .then((res) => {
        return res;
      });
  },
};

In StartUp
At ASP.NET CORE
ConfigureServices

//Enable CROS To allow access to the resource
            services.AddCors(o => o.AddPolicy("MyPolicy", builder =>
            {
                builder.AllowAnyOrigin()
                       .AllowAnyHeader()
                       .AllowAnyMethod();
            }));

 

In Configure

app.UseCors("MyPolicy");