Hi I just recently started running into this issue. I know it can be worked around by breaking the file into multiple parts.
I was contacting my webhost maybe they could of changed something but after narrowing the problem he said its smf thats causing the problem. He said that when you download the files that it loads file into the ram before downloading.
He did allow me to increase my memory_limit so that everything works again.
but is there some way to change how the downloading of attachments are handled??
btw I have the Encrypt stored filenames turned on.