This policy setting allows you to configure the list of low risk file types. If the attachment is in the list of low risk ...

This policy setting allows you to configure the list of low risk file types. If the attachment is in the list of low risk file types, Windows will not prompt the user before accessing the file, regardless of the file's zone information. This inclusion list overrides the list of high risk file types built into Windows and has a lower precedence than the High or Medium risk inclusion lists (where an extension is listed in more than one inclusion list.)

If you enable this policy setting you can specify file types which pose a low risk.

If you disable this policy setting Windows uses its default trust logic.

If you do not configure this policy setting Windows uses its default trust logic.