Per RFC 2046 §5.1.1, MIME multipart boundaries require CRLF (\r\n) as line separators, not bare LF (\n).

The boundary delimiter line is then defined as a line consisting entirely of two hyphen characters ("-", decimal value 45) followed by the boundary parameter value from the Content-Type header field, optional linear whitespace, and a terminating CRLF.

The CRLF preceding the boundary delimiter line is conceptually attached to the boundary so that it is possible to have a part that does not end with a CRLF (line break).

Credit to @viccie30.