/usr/lib/mime/packages/w3m is in w3m 0.5.3-15ubuntu0.2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | text/html; /usr/bin/w3m -T text/html %s; needsterminal; description=HTML Text; nametemplate=%s.html; priority=4
text/html; /usr/bin/w3m -I %{charset} -dump -T text/html %s; copiousoutput; description=HTML Text; nametemplate=%s.html; priority=3
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