How to post multipart/formdata using fetch in react-native?

You should have an upload function, which should look like this:

upload(url, data) {
  let options = {
    headers: {
      'Content-Type': 'multipart/form-data'
    },
    method: 'POST'
  };

  options.body = new FormData();
  for (let key in data) {
    options.body.append(key, data[key]);
  }

  return fetch(requestUrl, options)
      .then(response => {
        return response.json()
          .then(responseJson => {
            //You put some checks here
            return responseJson;
          });
      });
}

And you call it this way, sending the image blob path:

this.upload('http://exampleurl.com/someApiCall', {
  file: {
    uri: image.path,
    type: image.mime,
    name: image.name,
  }
}).then(r => {
  //do something with `r`
});