/etc/rplay/rplay.helpers is in rplay-server 3.3.2-14.
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 8 | # Sample rplay.helpers
#
# <regex> <audio-format> <application>
#
## MPEG
\.mp[1-3]$ linear16,44100,16,2,little-endian /usr/bin/mpg123 --stdout --quiet --rate 44100 --stereo -
\.ogg$ linear16,44100,16,2,little-endian /usr/bin/ogg123 -q -d raw -o byteorder:little -f - /dev/fd/0
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