In the /usr/mailscanner/etc/rules/ directory, edit filename.rules.rules and filetype.rules.rules (and the archives.filename.rules.rules and arvhies.filetype.rules.rules if you also want the same rules for scanning within zip archives). Add a line like this for each domain for which you don't want attachments to be scanned:
For filename.rules.rules add:
FromOrTo: domain.com /usr/mailscanner/etc/filename.no.rules.conf
For filetype.rules.rules add:
FromOrTo: domain.com /usr/mailscanner/etc/filetype.no.rules.conf
The files /usr/mailscanner/etc/filename.no.rules.conf and /usr/mailscanner/etc/filetype.no.rules.conf should simply contain the following line:
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allow . - -Make sure that you put tabs and not spaces between the parts of the above line.