MT#6243 fix multipart append_body_part function

Change-Id: If8d5840b70aedf6cbceb0739df684bf2c8c7ea0b
(cherry picked from commit 59ae20bf2eebba596ddb6932d8a18fbc608b4632)

Conflicts:
	debian/patches/series

(cherry picked from commit 173feba927736f0d5e04d8ed4086c00bb7ef66b3)