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If it is a rather large file, e.g. 650 MB, then it is most likely a CD image. If it is a file of smaller size, see at the end of this article. These MDF image files are created using a CD/DVD CD/DVD image file editor (and CD burner and CD/DVD backup tool) called MagicISO Maker.

MagicISO Maker can directly create, edit, extract, and burn ISO/MDF files. It also can convert almost all CD/DVD image formats to ISO/BIN/CUE, Nero (.NRG), and CloneCD (.CCD/. IMG/. SUB). With MagicISO, you can handle DVD image up to 10GB; make CD image files from CD/DVD-ROM; also can burn ISO files and other CD/DVD image files (BIN, IMG, CIF, NRG, BWI, VCD) to CD-R/RW; support loading boot image file exported by WinISO and UltraISO; burn CD image files in ISO9660, Joliet, and UDF format; edit properties of CD/DVD image files; it has the ability to correct volume serial number after editing original CD/DVD image file; search files or directories with wildcard characters in CD/DVD image file.

http://www.magiciso.com/download.htm