There is no central point to define file/mime extensions.
But file/mime extensions are used in so many points in asg:
Email Security -> SMTP -> AV
Email Security -> SMTP Profiles -> MIME b/w List, blocked ext.
Email Security -> POP3 -> AV
Web Security -> HTTP -> AV
Web Security -> HTTP -> AV
Web Security -> HTTP Profiles -> Filter Actions
Web Security -> FTP -> AV
Gregor Kemter