I think the OP should register at the Avast Support Desk and submit a bug report ticket about this issue. It's probably better to let the tech guys know about this issue so that it can be corrected in a later build of 5.0 than to go back to 4.8.
I mean, going back to 4.8 may solve your problem with these types of files/e-mail. But it's an issue that Avast should be made aware that you have stated Avast 4.8 can scan this type of file and 5.0 cannot. If it's not reported to the tech team, they should know about it because support for Avast 4.8 ends on December 31, 2010.
Jack