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File Format
Name ActiveMime
Extension(s) .mso
MIME Type(s) application/x-mso

Documented by phishme on GitHub, ActiveMime is an undocumented Microsoft file format often seen used to encode Microsoft Office Macros.

ActiveMime binary objects may reside in Microsoft Web Archive (MHT) files documented by Cyren.

There is little official documentation that can be searched for. The earliest references indexed by Google seem to come from 2012 but not much earlier.

There is more information and a further call for collaboration or corrections on what is documented on the Pishme GitHub README.

Magic Number

The structure is partially reversed engineered by phishme on GitHub. Once decoded from Base64 the magic number of the format is: 41 63 74 69 76 65 4d 69 6d 65 00 00 - 'ActiveMime' + null terminators.

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