Upload component - Type upload failure

Flow
1

I have implemented my second Vaadin-upload component. It is configured no-auto so as to be triggered by button press.
When I trigger the upload it attempts to do a post request to the server. Sometimes the request is sent as type xhr and it succeeds but most times it tries to send it as json, this then triggers a redirect to a get which fails (405).
Is it possible to force the request to be xhr and thereby avoid the failure case.

Regards
Mike

2

From what I can tell below ~250kb the software arbitarily chooses JSON or XHR to send the request. For me the JSON requests fail, and it seems to get stuck in that mode a lot. By adding additional header/form data into the request I have manged to force it into XHR mode irrespective of payload size.