/usr/share/mimelnk/chemical/x-mdl-rxnfile.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-mdl-rxnfile
Comment=MDL 2D Reaction-Data Representation Format
Comment[de]=MDL 2D Reaktionsdaten-Darstellungsformat
Comment[fr]=Format MDL de Représentation 2D de Données Réactionnelles
Icon=chemistry
Patterns=*.rxn;
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