How I can add to backup only *.au3 and *.exe files from a source?

Or you can use “Include Regular Expression”. I have made an example a few minutes ago for an exclusion of bak and tmp files. This can also be used for an include definition. Have a look here

--exclude="[.*\.(bak|tmp)$]"

can also be used as

--include="[.*\.(au3|exe)$]"