We currently have an issue with large file transfers, which we are looking in to. It seems to be network related, ie depends on what sort of devices are being used to manage the network, which is why we haven't picked it up before - all our testing worked!
Have you tried adding the following line to config.txt?
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed. Here's an example...
"My grief counseller just died, luckily, he was so good, I didn't care."