/etc/vdr/conf.avail/mplayer.conf is in vdr-plugin-mplayer 0.10.2-22.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# Command line parameters for vdr-plugin-mplayer
#
# For details see /usr/share/doc/vdr-plugin-mplayer/README and README.Debian
#
[mplayer]
--mount /usr/share/vdr-plugin-mplayer/mount
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