There is only one file type that uses the .MGK extension:
ImageMagick Configuration File
Configuration file used by ImageMagick, a free library used for creating and editing bitmap images; saved in an XML format and specifies settings for image formats, colors, modules, logging, and other components used by the software.
Example MGK filenames include colors.mgk, magick.mgk, modules.mgk, and log.mgk.
NOTE: Since many different software applications used ImageMagick, you may find MGK files within their installation folders.
Updated: August 12, 2012