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What is a .acsm file?

A .acsm file is an Adobe Content Server Message File. This one is a specialist format without an obvious everyday home.

How to open a .acsm file

The program that created it is by far the most reliable way to open it.
On Windows, use a hex viewer or the free TrID tool to identify what the file really is.
On Mac or Linux, running `file yourfile` in the terminal identifies most formats instantly.

The registry records this extension against Adobe Content Server Message File, so software from that family is the most likely match.

Identify the file first, then look for an export or Save As option in whatever program can read it.

When a .acsm file travels over the web, servers label it with this MIME type:

application/vnd.adobe.adept+xml

For archivists, developers and the merely curious: these are the PRONOM registry entries for .acsm.

Format name	Version	MIME type	PUID
Adobe Content Server Message File	—	application/vnd.adobe.adept+xml	fmt/871