In the previous release before v9.5, mail attachments like .txt, .html can still be scanned for spam, you want to know what the option "Text-based attachment scanning" make SMG 9.5 different.
With Symantec Messaging Gateway 9.5.x under GUI path:
Spam > Scan Settings > Email > "Text-based Attachment Scanning" option
the email attachments with the following extensions can be scanned for spam:
By default, this option is enabled for a new installation and disabled for upgrades. Enabling this option may result in slower performance of Symantec Messaging Gateway. When this option is disabled, Symantec Messaging Gateway does not use all scanning technologies for evaluating the attachments for spam.
In the previous release email attachments with .html, .htm, .txt, .xml can still be scanned, but not for .doc, .wps, .rtf. The reason is that attachments with text based attachemnts like .html, .htm, .txt, .xml is embeded as ASCII in the mail source, so the content can still be scanned for spam, but binary attachments like .doc, .wps, .rtf are embeded in the mail source encoded in base64 or Quoted Printable, only since 9.5 these files can be scanned.
Symantec Message Gateway 9.5