I'm developing software to sync contacts and calendars from a Zimbra server.

Since this is for a mobile platform I'd like to minimize the amount of data transferred. Since the contact image can be quite large relative to the rest of the contact info I'd like to only download it if it has changed. However there seems to be no way to find out the modification date of the image and the server seems to ignore the "If-modified-since" HTTP header.

Is there some other way to do this? If not, I would like to recommend it as an enhancement.