Hello Sachin,
four character file extension '.text' is set
lv_filename = 'AttachmentFilename.xlsx'. concatenate'&SO_FILENAME=' lv_filenameinto lv_text_line.
append lv_text_line to lt_att_head.
lo_document->add_attachment( exporting
i_attachment_type = 'XLS'
i_attachment_subject = 'Attachment'
i_attachment_size = '21'
i_att_content_text = lt_att_text
i_attachment_header = lt_att_head
).
different file type you are adding ..