Image

What is a .htm file?

A .htm file is a Hypertext Markup Language (version 2.0). This is an image format — a picture, drawing or graphic. Confusingly, this extension has been used by 10 different formats over the years — they're all listed below.

This extension has been used by more than one format

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

How to open a .htm file

GIMP (free) and IrfanView open a very wide range of image formats, including obsolete ones.
XnView MP is the best bet for unusual or legacy image types.
Your operating system's built-in viewer may work for mainstream formats.

The registry records this extension against Hypertext Markup Language, Hypertext Markup Language, Hypertext Markup Language and others, so software from that family is the most likely match.

How to convert a .htm file

Open it in GIMP or XnView and export as .png (lossless) or .jpg (smaller).

MIME type

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

text/htmlapplication/xhtml+xml

What's a MIME type?

The technical record

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