There are file types recommended to be blocked at the email gateway and are known to be a source of infection/malicious code.
It is recommended to block executable file types since infection risk is high. It may, for example, trigger malicious code or payload upon file click execution. Furthermore, executable file type attachment is not a good security practice.
Other file types may also be filtered (e.g. optional to block for pro-active protection) depending on your business needs, since these files can be normal attachments in most business industries and risk requires user interaction (e.g. the need to decompress, enabling of macro, or loading of disk image).