Answer
The standard Mail Reader effectively only surfaces text-based attachment content as separate application messages; binary attachments aren't picked up the same way. A working approach is to replace it with a custom IFS Connect mail reader (written against the Java mail-reader interface) that parses the email itself, converts every attachment's binary content to base64, and packages the whole thing (metadata plus base64 attachment data) as a single JSON application message. A routing rule can then read that JSON, and a PL/SQL block on the routing rule decodes the base64 back to binary and attaches the file to Document Management (DOCMAN) — letting an inbound email automatically create a record (e.g. a case) with its original PDF/image attachments intact.