This rule detects potential HTML smuggling attacks that utilize decimal encoding as part of their obfuscation strategy, making it difficult for traditional email security measures to identify malicious content. HTML smuggling typically involves encoding executable payloads into HTML files, which can then bypass security filters. The rule specifically looks for attachments with certain HTML file extensions or those with an unknown file type that are flagged as a default content type of 'application/octet-stream'. In addition, it checks for blocks of decimal encoding patterns (grouped numbers), which are indicative of hiding malicious content. The rule incorporates conditions to negate alerts from highly trusted sender domains unless they fail DMARC authentication, thereby reducing false positives from known good sources. Overall, the rule combines various methods of detection, including content and file analysis, focusing on suspicious identifiers within emails.