Too many fields when File Download

Loop at gt_file into ls_file.
  do 100 times.
     assign component sy-index of structure ls_file to <fs_field>.
     if  <fs_field> is assigned.
       concatenate ls_file
                         <fs_field>
                   into ls_file
            separated by cl_abap_char_utilities=>horizontal_tab.
    endif.
  enddo.
  endloop