Understand Image Upload logic in Flutter via multipart/form-data request?

Question

I'm currently trying to setup a multipart/form-data request in a flutter app which uploads a picture taken via the device's camera, using flutters image picker package.

To take the picture, I simply use:

final takenPicture = await ImagePicker().pickImage(
        source: ImageSource.camera
    );

fileForUpload = File(takenPicture.path);

I then upload the file according to the docs using:

var uri = Uri.https('example.com', 'create');
var request = http.MultipartRequest('POST', uri)
  ..fields['user'] = 'nweiz@google.com'
  ..files.add(await http.MultipartFile.fromPath(
      'pictures[]', fileForUpload.path,
    filename: 'sample.jpg',
      contentType: MediaType('image', 'jpg')));
var response = await request.send();
if (response.statusCode == 200) print('Uploaded!');

This now works. The problem I have is that, when you omit the filename and the contentType argument in the MultipartFile.fromPath constructor, the upload occasionally fails with the error message:

the file exists, but the PHP is_readable() function fails when passed the path to the file.

Pretty much like in this answer, it only worked once I added a random filename and content type to the request, as shown above; then the error stopped showing up.

I know that the Multipart class applies the application/octet-stream by default as the content-type of the image; is that maybe the reason? When I var_dump() the $_FILES variable on the server-side for the times where the above-mentioned upload error occurs, I can see that tmp_name is empty, which is probably the reason why the server-side cannot find the upload. When I add the above-mentioned filename and contentType parameters, the tmp_name property of all uploads is always present and the upload seemingly never fails.

Besides the fact that I would really like to understand why this happens, I would also like to know how you should actually set the filename + media type for an image that has been taken via the image picker package. To my understanding, that depends on the device if the image you've selected is png, jpg, heic, etc. Simply extract the mime type via the suffix from the filename, as the upload is then validated on the server-side anyways? Just because the docs do not really explain about the MediaType part.