This policy setting controls which types of files (determined by file extension) can be added to InfoPath forms as attachments.
If you enable this policy setting and "Prevent users from allowing unsafe file types to be attached to forms" is Disabled or not configured, you can specify file type extensions to remove from the standard list of prohibited file types in InfoPath, thereby allowing users to attach files of the specified types to forms.
Important - If "Prevent users from allowing unsafe file types to be attached to forms" is enabled, enabling this policy setting does nothing: any extensions specified here will not be allowed as attachments to forms. Conversely, if there is a legitimate need to allow users to attach files of unsafe types to forms, the "Prevent users from allowing unsafe file types to be attached to forms" setting must be disabled in addition to enabling this policy setting and specifying the file types to allow.
If you disable or do not configure this policy setting, users can attach any type of file to forms except potentially unsafe files that might contain viruses, such as .bat or .exe files.