(General Reply)

If we leave aside the discussion whether mIRC is or is not a filesharing program the question still stays.

How mIRC actually handles files?

For example $isfile(C:\song) returns $true
but /splay [-p] C:\song returns unable to play 'C:\song'
(C:\song is renamed C:\song.mp3)
PS. just for records the song plays just fine if it has extension so dont blame the broken soundfile blush

I haven't messed around with it that much to tell whether some other functions fail as well with files without extensions.
$read and /play seem to work but /splay was no-go for me at least.

echo -a $signature