The types of attachments we see for malware-laden email
These days we block some attachment types, but of course not all of them. We also use our commercial anti-spam package to detect and reject malware that isn't already rejected outright as a bad attachment type. This raises the obvious interesting question of what attachment types that malware comes in. Unfortunately I can't answer exactly that question from our logs, but I can see what attachment types we see in email that is rejected because it contains malware.
(This difference matters only if a malware email contains multiple attachments, possibly with multiple pieces of malware. When this happens, our commercial anti-spam system doesn't log information that would let us identify which particular attachment has the detected malware.)
In the past 19 days of logs, we rejected 440 messages because they were identified as containing malware. A few of these messages had attachments without MIME filenames; all of the attachments were detected as either Microsoft Word files or images (mostly PNGs with a couple of GIFs). The vast majority had MIME filenames, and the (claimed) extensions on these are as follows:
241 .doc 95 .jar 52 .zip 24 .html 6 .htm 6 .gz 3 .7z .rar 1 .z .xls .rev .psx .pdf
Most of the .zip files contained .doc files (26) or .jar files (12). Ten of the .zip files were actually given the MIME type application/msword (despite the .zip extension in their MIME filename) and contain the file extensions '.bin .png .rels .xml none'. Some Internet searches suggest that the .bin file extension here is the giveaway marker of Word macros being embedded in the file, and we certainly saw a bunch of .doc files that had .bin files inside them (it looks like 69 out of the 241 .doc files, so by no means all or even a majority but certainly a significant portion).
Having gone through this stats gathering exercise, my overall view is that there's nothing terribly surprising here. Microsoft Word and Java/JARs are a big attack vector and HTML files appear to be a side door to sneak some attacks and phishing through where they might be blocked as an actual HTML email message.
(The obvious question to ask next is if our users get very many legitimate email messages that have Word files with embedded macros.)