Using GUI upload to attach a file to email in SAP

You have to use parameters correctly in the function. You have to import the file length to an integer value.

CALL METHOD cl_gui_frontend_services=>gui_upload_file
...
    IMPORTING
    filelength = fleng_i
...

Next is to write to a character type variable and add parameter i_attachment_size to a class of add attachment.

CALL METHOD w_document->add_attachment
EXPORTING
    i_attachment_size = fleng
 ...