CALL FUNCTION ’KD_GET_FILENAME_ON_F4’ 
   EXPORTING 
    MASK = ’,*.txt,*.*’ 
    STATIC = ’X’ 
  CHANGING 
    FILE_NAME = LV_FILE.

Method 2 

     CALL FUNCTION ’F4_FILENAME’
       EXPORTING
         program_name  = syst-cprog
         dynpro_number = syst-dynnr
         field_name    = ’ ’
       IMPORTING
         file_name     = LV_FILE.