ActiveMime
Unknowndocument
ActiveMime is a Microsoft file format which can be used to encode Office Macros. It is undocumented but the file structure contains two parts. A header containing magic bytes and data about the file such as size and metadata. Followed by a zlib compressed data block.
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| Extensions | .mso |
|---|---|
| MIME types | application/x-mso |
| Signatures | 4163746976654d696d65 at byte 0 (ActiveMime Object) |
| PRONOM | fmt/1915 |
| Wikidata | Q28600288 |
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From Just Solve the File Format Problem (File Formats Wiki), PRONOM, Wikidata. Updated 2026-10-24.