/usr/share/frei0r/html/files.js is in frei0r-plugins-doc 1.6.1-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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var files = [ [ "frei0r.h", "frei0r_8h.html", "frei0r_8h" ] ];