/usr/share/application-registry/texmacs.applications is in texmacs-common 1:1.0.7.18-1.
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 | texmacs
command=texmacs
name=GNU TeXmacs
can_open_multiple_files=true
expects_uris=false
requires_terminal=false
mime_types=text/texmacs;text/english;text/plain;text/x-tex
|