The following commit: https://github.com/mikakoivisto/liferay-portal/commit/07191c476777bb1baa53a6bf070debc174fcbd1c has led to IE7/IE8 browsers being unable to download files from the document library due to their response to the private cache control headers on our SSL Liferay installation:
Please refer to http://support.microsoft.com/kb/316431 for a write up by Microsoft on the by-design behavior.
We noticed this change in behavior because we just upgraded from 6.0.12 EE to 6.1.20 EE GA2.
Our current workaround is to override the behavior of the PortletResponseUtil and ServletResponseUtil classes' setHeaders(request, response, fileName, contentType) method to switch those headers over to being:
only for IE browsers that are requesting content from the Documents directory. It does the trick but might not be the most elegant solution at this point.