Collector http://www.zope.org/Collectors/Zope/1797 : 304 responses require...

Collector http://www.zope.org/Collectors/Zope/1797 : 304 responses require collaboration from http accelerated cache manager.  (see also http://www.zope.org/Collectors/Zope/683).

lib/python/OFS/Image.py

@@ -365,6 +365,12 @@ class File(Persistent, Implicit, PropertyManager,
 
         if self._if_modified_since_request_handler(REQUEST, RESPONSE):
             # we were able to handle this by returning a 304
+            # unfortunately, because the HTTP cache manager uses the cache
+            # API, and because 304 responses are required to carry the Expires
+            # header for HTTP/1.1, we need to call ZCacheable_set here.
+            # This is nonsensical for caches other than the HTTP cache manager
+            # unfortunately.
+            self.ZCacheable_set(None)
             return ''
 
         if self.precondition and hasattr(self, str(self.precondition)):