OutOfFox

$upper(%_extension%)/
$set(_aartist,$substr($if2(%albumartist%,%artist%),0,1))
$if($in(0123456789,%_aartist%),#,$if($not($in(ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz,%_aartist%)),#,$upper(%_aartist%)))
/$if2(%albumartist%,%artist%) (%musicbrainz_albumartistid%)/
%album% (%musicbrainz_albumid%)/
$if($gt(%totaldiscs%,1),$if($gt(%totaldiscs%,9),$num(%discnumber%,2),%discnumber%)-,)$num(%tracknumber%,2) - %title%

Line by line:
Extension/
Set variable to first letter of artist.