Cannot upload image using dio post request - flutter

Question

I am trying to upload a photo with caption to REST API. But since it is my first time, I am getting some exceptions. My code for uploading the image is shown below:
Dio dio = new Dio();
dio.options.headers['content-Type'] = 'multipart/form-data';
dio.options.headers['Authorization'] = 'Token ${box.get('token')}';
FormData formData = FormData.fromMap({
"author": box.get('user_id'),
"description": _caption,
"image": MultipartFile.fromFileSync(_image.path,
filename: _image.path.split(Platform.pathSeparator).last)
});
dio
.post("$baseURL/posts/create/",
data: formData,
options:
Options(method: 'POST', responseType: ResponseType.plain))
.then((response) => print(response))
.catchError((error) => print(error));
After I run this on my device and try to upload image, it directs me to dio.dart package line throw assureDioError(e, options);
If you have any knowledge with uploading images using dio, kindly help solve it.
UPDATED: body of json
{
"author": null,
"description": "",
"image": null
}
UPDATED: I tried with another service uploading image (not dio)
// string to uri
var uri = Uri.parse("$baseURL/posts/create/");
// create multipart request
var request = http.MultipartRequest("POST", uri);
// add headers with Auth token
Map<String, String> headers = {
"Authorization": "Token ${box.get('token')}",
"Content-type": "multipart/form-data"
};
request.headers.addAll(headers);
// add fields
Map<String, String> fields = {
'author': box.get('user_id').toString(),
'description': _caption,
};
request.fields.addAll(fields);
// multipart that takes file
var multipartFile = http.MultipartFile(
'file',
_image.readAsBytes().asStream(),
_image.lengthSync(),
filename: _image.path.split('/').last,
contentType: MediaType('image', 'jpeg'),
);
// add file to multipart
request.files.add(multipartFile);
print(request.toString());
// send
var response = await request.send();
print(response.statusCode);
// listen for response
response.stream.transform(utf8.decoder).listen((value) {
print(value);
});
I have tried uploading images with the above code as well. But this returns me the error saying "image":["No file was submitted."]