Document

What is a .fm file?

A .fm file is an Adobe FrameMaker Document (version 9.0). This is a document format, so it holds text and layout meant to be read. Confusingly, this extension has been used by 12 different formats over the years — they're all listed below.

This extension has been used by more than one format

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

How to open a .fm file

Try LibreOffice (free, Windows/Mac/Linux) — it opens an enormous range of current and historic document formats.
Microsoft Word and Google Docs handle most mainstream document types.
If nothing opens it, open it in a text editor: many older document formats have readable text inside.

The registry records this extension against Adobe FrameMaker Document, Adobe FrameMaker Document, FileMaker Pro Database and others, so software from that family is the most likely match.

How to convert a .fm file

Once it opens in LibreOffice or Word, Save As .docx or .pdf to get it into a format you can use anywhere.

MIME type

When a .fm file travels over the web, servers label it with one of these MIME types:

application/vnd.framemakerapplication/x-filemaker

What's a MIME type?

The technical record

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