Microsoft Office file formats dating from 2007 are now being actively exploited to hide the presence of macros and distribute malware. This blog was written by Matthew Molyett with contributions from Martin Lee. Macros have been used since the mid 1990s to spread malware and infect systems. The OfficeOpen XML (OOXML) standard integrated in MS Office 2007 removed this ambiguity. Each of the four file formats supported by MS Word have unique MIME types. Only two, those associated with DOCM and DOTM, can save or run macros.”]
Source: https://blog.talosintelligence.com/2016/08/macro-intruders-sneaking-past-office.html