Something

  //parsing CommaExts and passing each ext to WMPRegMultimediaExts
  S := 1;
  for I := 1 to Length(CommaExts) do
    if CommaExts[I] = ',' then
    begin
      RegExt := Copy(CommaExts, S, I - 1);
      S := I + 1;
      WMPRegMultimediaExts(RegExt, PerceivedType, MIMEType, ExtDesc);
    end
    else if I = Length(CommaExts) then
    begin
      RegExt := Copy(CommaExts, S, Length(CommaExts));
      WMPRegMultimediaExts(RegExt, PerceivedType, MIMEType, ExtDesc);
    end;