I had a form that accepted a file and some fields and I wanted to send this data to the server through the Fetch API, like this (React code):

<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>

I had a problem. The files data was not actually sent to the server.

The solution was this: you must NOT set the headers.

I did set the multipart/form-data header as that’s what you do with files upload, but apparently that’s what is breaking the file upload through fetch.

Remove the headers from the fetch request and things should be working.