Something

    if (WMPCount + Length(Exts) >= WMPOpenMaxChar)
    begin
      Exts := Exts + ';';
      P := 1;
      for I := 1 to Length(Exts) do
      begin
        if (WMPCount + I) > WMPOpenMaxChar then Break;
        if Exts[I] = ';' then P := I;
      end;
      Delete(Exts, P, Length(Exts) - P + 1);
    end;

    if Exts <> '' then
    begin
      WMPCount := WMPCount + Length(Exts) + 1;