Outbound Proxy..... Attaching multiple attachments

Hi Gurus,

I want to attach multiple files with my ABAP outbound proxy for Purchase Requisitions. I have done the single attachment but unable to attach multiple files. I am using the following code for attachment.

TYPES:
    BEGIN OF gty_file_data,
      entry TYPE x LENGTH 255,
      END OF gty_file_data.

DATA:

  gr_consumer_proxy     TYPE REF TO   zco_consumer_proxy,

  gr_proto_attachment   TYPE REF TO   if_wsprotocol_attachments,

  gr_attachment         TYPE REF TO   if_ai_attachment,
  gt_attachments        TYPE          prx_attach,

  gt_file_data          TYPE TABLE OF gty_file_data,

  gv_attachment_xstring TYPE          xstring.

FIELD-SYMBOLS:

  <gfs_file_data> TYPE gty_file_data.

TRY.

    CALL FUNCTION 'GUI_UPLOAD'

      EXPORTING
        filename = 'D:\Temp\TestDoc.pdf'
        filetype = 'BIN'
      TABLES
        data_tab = gt_file_data.

    IF sy-subrc NE 0.

        " Error handling logic

      ENDIF.

    LOOP AT gt_file_data ASSIGNING <gfs_file_data>.
      CONCATENATE gv_attachment_xstring

                  <gfs_file_data>-entry

             INTO gv_attachment_xstring

             IN BYTE MODE.
      ENDLOOP.

   CREATE OBJECT gr_consumer_proxy.

    gr_proto_attachment ?=

      gr_consumer_proxy->get_protocol( if_wsprotocol=>attachments ). 

    gr_attachment =

      gr_proto_attachment->get_attachment_from_binary(

                             data = gv_attachment_xstring

                             type = if_ai_attachment=>c_mimetype_pdf

                             name = 'AttachmentDocument' ).
    APPEND gr_attachment TO gt_attachments.
    gr_proto_attachment->set_attachments( gt_attachments ).

  CATCH cx_ai_system_fault INTO gx_system.

    " Exception handling logic

  ENDTRY.

Actually in FM Gui_upload, if give specific location to input parameter file name, it works.

CALL FUNCTION 'GUI_UPLOAD'

      EXPORTING
        filename = 'D:\Temp\TestDoc.pdf'
        filetype = 'BIN'
      TABLES
        data_tab = gt_file_data.

But if i want to pass multiple files locations inside loop, it gives error "File is not available". Like this

loop at ftab into waf.

   CALL FUNCTION 'GUI_UPLOAD'

       EXPORTING

         filename =  waf-filename    

         filetype = 'BIN'

       TABLES

         data_tab = gt_file_data.

     IF sy-subrc NE 0.

         " Error handling logic

endloop.

       ENDIF.

can anybody tell me how can i use this FM for multiple attachment or is there any other wau to attach multiple files in outbound abap proxy.

Thanks

Regards