R650 Firmware update fails with size error(?)

We have a bunch of R650 APs which have factory 200.10.xx unleashed firmware installed. I've tried to upgrade to the newest firmware, but the latest the AP accepted was 200.12.10.105.129. Every newer firmware got stuck in "processing..." after the upload. I can see in the developer console from the browser that the upgrade file is rejected from the AP due to size contraints:
/admin/_upload.jsp?request_type=xhr 413 (Request Entity Too Large)