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What is a .zfo file?

A .zfo file is a ZFO (Form) File. This one is a specialist format without an obvious everyday home. Confusingly, this extension has been used by 4 different formats over the years — they're all listed below.

This extension has been used by more than one format

4 different formats have claimed .zfo over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .zfo 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 ZFO (Form) File, ZFO (Message) File, ZFO (Sent Message) File and others, so software from that family is the most likely match.

How to convert a .zfo file

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

MIME type

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

application/vnd.software602.filler.form-xml-zip

What's a MIME type?

The technical record

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