/usr/share/mime-info/gretl.mime is in gretl-common 1.9.6-1build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | application/x-gretldata
ext: gdt
application/x-gretlsession
ext: gretl
application/x-gretlscript
ext: inp
|