Follow up point please. In my Web Shield exceptions, there are various MIME types entered (by default presumably) audio/* image/gif, image/png, text/css, video/. Why would they be there if I have ticked to scan all files in web scanning? Would the video/ and audio/* entries mean that in my original question, video and audio files I download from sites would not be scanned in real time?
On the first page of Search results of this forum, I found the below. Perhaps that will help answer your question.
http://forum.avast.com/index.php?topic=14935.msg126161#msg126161
I hope this helps you. 
Indeed yes, thankyou. I entered six or seven different phrases in the Search box and failed to find that.
The only thing I entered was … mime. 