You should set 'scan all files' by default, because of many vulnerable extenstions ! As an example - years before we excluded *.txt files from scanning,
Hackers do know these security gaps and use it ! Also *.tmp files are used as infection today, every gap is a way into the computer for hackers,
I would recommend to scan all files with on access and use the Virusscan 8.8 with chaching of as 'clean scanned' files.
To use 'Scan default extension' is to dangerous today. I have seen McAfee has an detection :
as Performance optimisation there are 'low Risk' and 'high Risk' Processes usable to exclude specific processes and extensions.
see https://community.mcafee.com/thread/4438 for best Practise and the search for the Virusscan 8.8 Best Practise Guide