Main class: spam
Sub class: pattern
Attachment is being blocked as a Potentially Unwanted Attachment (PUA)
PUA restriction controls whether attachments that are considered dangerous or otherwise unwanted are rejected, for example compressed executable files (e.g. UPX packers), password tools, network tools, peer-to-peer clients, remote access applications, system tools, spying tools, and documents containing scripts.
Login into the spam expert -> click on the Dashboard
Scroll down t the right side click on the Attachment restrictions
Uncheck Block potentially unwanted attachments and save it.