怎么改请求里的response headers里的contentType为application/json

[JUNHAN12138]

我在umi框架里写的api接口怎么改请求里的response headers里的contentType为application/json

`import type { UmiApiRequest, UmiApiResponse } from "umi";
import fs from "fs";
import path from "path";

export default async function (req: UmiApiRequest, res: UmiApiResponse) {
switch (req.method) {
case "GET":
try {
const { id } = req.body;
const mdxPath = path.join(
process.cwd(),
"src/mdxFile",
${20231127}.mdx
);

    const mdxContent = fs.readFileSync(mdxPath, "utf8");

    res
      .status(200)
      .header("Content-Type", "application/json")
      .json({ content: mdxContent });
  } catch {
    res.status(500).header("Content-Type", "application/json").json({
      result: false,
      message: "报错",
    });
  }
  break;

}
}`

请求的接口response一直是这个

导致fetch的res.json()报错，