File attachments in email are common threat vectors for malware. Executable files are not the only file type that can include threats. For example, Microsoft Office documents (e.g. doc, docx, xlx, xlsx, ppt, pptx, etc) can contain macros or scripts that include threats.
Blocking specific file extensions or types can limit exposure to risk.
Blocking files extensions does not remove all risk of malicious files being received.
Blocking file extensions can also block valid files.
Most Symantec Mail Security products have the ability to block attachments by file extension. Some products can also block the true file type, even if the file has been renamed (e.g. foo.exe to foo.txt).