|Situation||What is the attachment type Undecipherable Attachments?|
|Solution||This article explains what Undecipherable Attachment for attachment type does|
If you create a filter with this option, it will come into effect when the engine module responsible for decomposing an email into parts does not know what an attachment is supposed to be. This can happen when the MIME part doesn't have a filename, or even a MIME type, or it is broken enough that these things don't make sense.
Even when the engine module is able to extract the attachment (or the file inside an archive), if these don't have any recognizable features that binary detection can latch on to, rules with this option set will be triggered.