Is there a file with such definitions and/or a directory of mimetype-files?
AFAIK /usr/share/mime/globs file contains a list of all mime-types (with extensions) recognized by the system.
But right-clicking a file in GnomeCommander, and then Properties -> Change will bring up PGnomeMimeType window with 'mime-type' entry already filled; the only task is to choose a suitable .desktop file.
BTW, G. C. uses /root/.local/share/applications/defaults.list file to store the associations.
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