/usr/share/mimelnk/chemical/x-mmcif.desktop is in chemical-mime-data 0.1.94-6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Encoding=UTF-8
Type=MimeType
MimeType=chemical/x-mmcif
Comment=MacroMolecular Crystallographic Information File Format
Comment[de]=MacroMolecular Dateiformat Kristallographisches Austauschformat
Comment[fr]=Format d'Échange Cristallographique pour les Structures de Macromolécule
Icon=chemistry
Patterns=*.mcif;
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