/usr/lib/mime/packages/xchm is in xchm 2:1.23-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | application/x-chm; /usr/bin/xchm '%s'; edit=xchm '%s'; test=test "$DISPLAY" != ""; nametemplate=%s.chm; description="Compiled HTML Help"
|