Fix uploading files using fetch and multipart/form-data

<form
  encType='multipart/form-data'
  action='/api/post'
  method='POST'
  onSubmit={async (e) => {
    e.preventDefault()

    if (!title) {
      alert('Enter a title')
      return
    }
    if (!content && !image) {
      alert('Enter some text in the post')
      return
    }

    const body = new FormData()
    body.append('image', image)
    body.append('title', title)
    body.append('content', content)

    const res = await fetch('/api/post', {
      body,
      method: 'POST',
      headers: {
        'Content-Type': 'multipart/form-data',
      },
    })
  }}
>
...
</form>