HMK is an extension that is specific to files associated with a greeting card maker called Hallmark Card Studio. HMK files are used to store projects made in the program. The user has the option of placing his own photos in such projects, e.g. when creating calendars, postcards or album pages.

Hallmark Card Studio contains approximately 11,000 sample projects and has a library of over 13,000 graphics. The software was developed by Hallmark Software.