Unknown errors when creating invoices

[IiroP]

Laskugeneraattori backend has quite strict rate limiting (triggered by 5+ requests in short timeframe). When sending requests that have attachments that are bigger than ~500 kB (exact amount unknown), the backend terminates request before all content has been send. This can be seen when using curl:

* HTTP error before end of send, stop sending
* abort upload after having sent 524085 bytes

If this situation is triggered with Laskugeneraattori frontend, it leads to "Jotain meni pieleen" error page, with server log showing: