[SimonItaly]

You're right, just tested on a blank gamemode and your code worked good.
It must be something related to the creation of the dialog.

pawn Код:

COMMAND:guidatest(playerid, params[])
{
    new File: file = fopen("Guide/Guide_Main.info", io_read),
        string[64], longstring[600];
    if(file)
    {
        while(fread(file, string))
        {
            strcat(longstring, string);
            strcat(longstring, "\n");
        }
        fclose(file);
        ShowPlayerDialog(playerid, 999, DIALOG_STYLE_LIST, "Guida", longstring, "Seleziona", "Annulla");
    }
    return 1;
}

I've tested this on the same filterscript and the commands (/test, /guidatest) printed two different results with the same dialog list.