It is impossible to detect .bat and .cmd files by format, because these are regular plain text files.

If you want to block attachments, you can only configure detection of these files by masks: *.bat, *.cmd.

Please check the section "Configuring the general settings and conditions of rules" of the sites https://support.kaspersky.com/KS4Exchange/9.6/en-US/166855.htm

Add a condition for the Attachment filtering rule and select File name mask instead of File format and then add *.bat or *.cmd to the list.