https://www.bleepingcomputer.com/news/security/html-attachments-remain-popular-among-phishing-actors-in-2022/

HTML files remain one of the most popular attachments used in phishing attacks for the first four months of 2022, showing that the technique remains effective against antispam engines and works well on the victims themselves.

HTML (HyperText Markup Language) is a language that defines the meaning and structure of web content. HTML files are interactive content documents designed specifically for digital viewing within web browsers.