iOS Mail attachment download behaviour when connected to Microsoft Exchange?

When is an eMail attachement automatically downloaded in iOS? From my expierience iOS downloads an attachment up to the size of about 50 kB automatically when on a 3G network, on a WLAN larger files are downloaded automatically (up to around 150 kB).

However I cannot find this documented by Apple and I have users complaining with high roaming bills that tell me big attachments get downloaded automatically.

Our users are connected to Microsoft Exchange 2010 by ActiveSync.

The problem is difficult to reproduce and I'm looking for ways to nail or solve this?

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.