Workaround buggy mime_content_type on php 5.2.0+etch10 (not tested on other version).

Workaround buggy mime_content_type on php 5.2.0+etch10 (not tested on other version).

In this version, mime_content_type as a fallback to text/plain instead of
application/octet-stream, leading to invalid email attachments (Closes #802).

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>