iOS Mail attachment download behaviour when connected to Microsoft Exchange?
i did some testing with a colleague, and here's what we've found:

if you open an email while connected to wifi, an individual attachment of up to 1027kb will auto-download, and be available for offline use.
if you see an email has been received, but do not open it, an individual attachment of up to 799kb will auto-download, and be available for offline use.
we still have not found the lower limit for emails with multiple attachments, though we do know it's lower than 613kb (we tried an email with two attachments: 365kb + 248kb and NEITHER auto-downloaded).
JPEGs (and possibly other image formats) have a higher auto-download size limit than other attachments, but we were not concerned with them, so we didn't test for their upper limit.