Question about Microsoft xlsx/docx/pptx files

Hi,
My distro is still running amavisd-new-2.10.1-6 and I am having issues 
with some Microsoft Office documents.  I have blocks for .bin and .zip 
attachments but nothing for .xlsx/pptx/docx and some xlsx files are 
being tripped up by the zip filter. I have read that I can whitelist 
them with:
[qr'\.(xlsx|docx|pptx)$'=>0]

in the $banned_filename_re parameter rather than allow zip files. I've 
tried searching and have not been able to work out the different options 
for the "=>0" bit of the parameter.

Does this approach have an disadvantages? Does it just mean these files 
will not be blocked but the rest of the message will be checked and 
these files will still be checked for viruses or does it have wider 
reaching consequences?

Regards,

Nick