How to make a POST Request in Flutter to send an mp4 file? (Without non-core APIs)

Question

I want to understand and know how to make a multiPart request with an mp4 file attached. While I understand there are posts that speak of dependencies such as dio or flutter_upload to do this, I don't want to cheat myself in knowing the why of things. So how do I accomplish this task? I keep getting the following error when making a request:

Error Snippet:

flutter: Failed due to: Unsupported Media Type and status code: 415

My endpoint only accepts Content-Types of application/JSON, but my app still threw when I tried to refactor my request to send json. I understand Dart requires data to be encoded before making a request but I am confused about how I do this with a MultipartFile instance when I get the following error: The argument type 'String' can't be assigned to the parameter type 'MultipartFile'. Finally, I refactored my function to reflect my latest attempt to send the correct format (despite no encoding involved :D).

Code Snippet:

Future<void> createAudioUpload(String fileUrl) async {
  Uri ourUri = Uri.parse(
      'https://[redacted]');
  final request =
      http.MultipartRequest('POST', ourUri); // creates our req instance

  final file = await http.MultipartFile.fromPath('audio', fileUrl,
      contentType: MediaType('audio', 'mp4')); // points a ref to file

  request.files.add(file); // adds our file to our request instance
  final response = await request.send(); // sends our request to the endpoint!

  if (response.statusCode == 200) {
    print('Success, $response');
  } else {
    print('Failed due to:${response.reasonPhrase}');
  }
}

Code Snippet V2: (sending data as bytes)

Future<void> createAudioUpload(String fileUrl) async {
  Uri ourUri = Uri.parse(
      '[redacted]');
  final request =
      http.MultipartRequest('POST', ourUri); // creates our req instance

  File ourFile = File(fileUrl);
  List<int> fileBytes = ourFile.readAsBytesSync();
  // String base64File = base64Encode(fileBytes);

  final file = http.MultipartFile.fromBytes('flutter_sound', fileBytes,
      filename: fileUrl.split("/").last,
      );

  request.files.add(file); // adds our file to our request instance
  final response = await request.send(); // sends our request to the endpoint!

  if (response.statusCode == 200) {
    print('Success, $response');
  } else {
    print('Failed due to: ${response.reasonPhrase} and status code: ${response.statusCode} ');
  }
}

Questions I want to understand (after I referenced flutter docs):

1. What is supposed to be the first positional argument of MultipartFile.fromPath
2. What is the correct syntax to pass for application/json? would the arguments be passed as "application", "json" for MIME main and sub type per the docs at https://pub.dev/documentation/http_parser/latest/http_parser/MediaType/MediaType.html ?

Quick Note

1. The data passes through an API gateway and is eventually stored in an S3 bucket.
2. I referenced https://pub.dev/documentation/http/latest/http/MultipartRequest-class.html for constructing the code.
3. If there is known documentation how to convert the audio file to bytes before sending that is also great!

Any help is appreciated.