What is a .idml file?

A .idml file is an InDesign Markup Language Package. This is an archive — a container holding other files, usually compressed.

How to open a .idml file

7-Zip (Windows) or Keka / The Unarchiver (Mac) extract most archive formats, including old ones.
On Linux, try `7z x`, `unar` or Ark.

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

Extract the contents, then re-compress as .zip if you need to share them widely.

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

application/vnd.adobe.indesign-idml-package

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

Format name	Version	MIME type	PUID
InDesign Markup Language Package	—	application/vnd.adobe.indesign-idml-package	fmt/1184