Creating signed request for S3 upload using Django

I have a simple django app on heroku for uploading images and it works as expected when accessed from desktop but fails to sign the requests for upload to AWS S3 when I access the site on mobile.

The signing code is shown below:

def sign_s3(request):
    request = request.GET
    S3_BUCKET = os.environ.get('AWS_STORAGE_BUCKET_NAME')

    file_name = request.get('file_name')
    file_type = request.get('file_type')

    s3 = boto3.client('s3')

    presigned_post = s3.generate_presigned_post(
    Bucket = S3_BUCKET,
    Key = file_name,
    Fields = {"acl": "public-read", "Content-Type": file_type},
    Conditions = [
        {"acl": "public-read"},
        {"Content-Type": file_type}
    ],
    ExpiresIn = 3600
    )

    return HttpResponse(json.dumps({
    'data': presigned_post,
    'url': 'https://%s.s3.amazonaws.com/%s' % (S3_BUCKET, file_name)
    }))

I set everything in the S3 bucket to public and have the CORS set as

<?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
<CORSRule>
    <AllowedOrigin>*</AllowedOrigin>
    <AllowedMethod>GET</AllowedMethod>
    <AllowedMethod>POST</AllowedMethod>
    <AllowedMethod>PUT</AllowedMethod>
    <AllowedHeader>*</AllowedHeader>
</CORSRule>
</CORSConfiguration>

This is a sample of the javascript used to send the request. I have realised it doesnt seem to send at all when its on mobile. The status code comes out as 0.

function getSignedRequest(file){
  var xhr = new XMLHttpRequest();
  xhr.open("GET", "/sign_s3?file_name="+file.name+"&file_type="+file.type);
  xhr.onreadystatechange = function(){
    if(xhr.readyState === 4){
      if(xhr.status === 200){
        var response = JSON.parse(xhr.responseText);
        uploadFile(file, response.data, response.url);
      }
      else{
        alert("Could not get signed URL.");
      }
    }
  };
  xhr.send();
}

uploadFile is just a separate function that finally posts the image.

I cant tell what could be causing this difference. How can I get this working?