What function does the filter option ‘Undecipherable attachments’ perform?

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, then 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. 

Have more questions about filters? Check out some of the below articles:

Quarantine actions

Multi-language anti-spam filter