为什么我的fetch body在Google Chrome扩展中总是返回null

{
  "manifest_version": 2,
  "name": "My Extension",
  "version": "1.0",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "http://*/",
    "https://*/"
  ],
  "optional_permissions": [
    "http://*/",
    "https://*/"
  ],
  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "content_scripts": [
    {
      "matches": ["http://*/*", "https://*/*"],
      "js": ["content.js"]
    }
  ]
}

const formData = new FormData();
formData.append('key1', 'value1');
formData.append('key2', 'value2');

fetch('https://example.com/api', {
  method: 'POST',
  body: formData
})
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error(error));

fetch('https://example.com/api', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({ key1: 'value1', key2: 'value2' })
})
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error(error));