Hi Jo,
I'm using PHP 5.4.4 and testing your newest commits. After the POST to import.php, I immediately see
GET import_status.php with this JSON response:
{"id":"4fe8b2f19bc80","finished":true,"percent":100,"total":0,"complete":0,"plugin":"PHP_SESSION_UPLOAD_PROGRESS"}
Then, another GET to import_status.php which does not return.
Meanwhile, I see in my upload directory the file still being received.