I’m not, but maybe this will help? https://technet.microsoft.com/en-us/library/bb332342(v=exchg.150).aspx

It’s pretty easy to Google and find that pretty much all the major vendors recommend the same file exclusions, so IMHO it would be wise to do the same for Avast Business

Remember though that mail is not scanned through the MTA with the free Avast Business; you’ll need to subscribe to the Avast Premium Business Security for that level I think.