File with an .MDX extension contains an image of an optical drive – CD or DVD, on which all of the information, the metadata describing the disc and the disc data itself, are contained within a single file. This format was introduced in order to substitute two types of files – MDS and MDF – with a single MDX file.

The extended media description files – MDX, are used by the Deamon Tools software, in order to create images of existing discs, and in order to emulate drives in a system. MDX files are also referred to as the extended multimedia data files – Media Data eXtended.