F-Secure researchers have tracked the top spam-related attachments and campaigns used so far in 2019. ZIPs, PDF, and MS office files were more commonly used in huge spam campaigns than any other type attachment. In February and March, there were huge campaigns using ZIP files to send out GandCrab ransomware, and DOC and XLSM files to distribute Trickbot banking trojan. Disc image files are increasingly being used to spread malware, according to researchers. In March, a phishing attack that was making use of PDF files to target American Express customers.
Source: https://threatpost.com/threatlist-top-5-most-dangerous-attachment-types/144635/