/usr/share/GNUstep/GNUMail.app/GNUMail.desktop is in gnumail.app-common 1.2.2-1.1+deb9u1.
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=Application
Categories=X-GNUstep;
Name=GNUMail
Comment=A GNUstep mail client.
Icon=/usr/lib/GNUstep/Applications/GNUMail.app/Resources/GNUMail.tiff
Exec=/usr/bin/openapp GNUMail
FilePattern=GNUMail.app;GNUMail;
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