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Fix uploading files using fetch and multipart/form-data

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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):

  onSubmit={async (e) => {

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

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

    const res = await fetch('/api/post', {
      method: 'POST',
      headers: {
        'Content-Type': 'multipart/form-data',

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 thatโ€™s what you do with files upload, but apparently thatโ€™s what is breaking the file upload through fetch.

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

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