Anyway, whenever I try to save "Articulo_y_descripccion.xls" as a csv format file, I got a popup message warning about a possible problem of compatibility and giving the option to save it with other name...I don't truly understand why is this...
This is normal Excel warnings. You have nothing to lose in proceeding to save the file in .csv format. Since it is a "Save As" action your original workbook remains intact and you can always revert to it.