MMC - Microsoft Media Catalog
This is a file which may contain collections of images. For instance a clip art gallery or other media may be organized in an MMC file. It is typically stored in the 'Application Data]MicrosoftMedia Catalog' folder. One example of a MMC file is the 'Artgal50.,,c' file. These files may contain keywords, images and thumbnails which are utilized by the Clip Gallery applet.