Workaround libmagic bug

Hi Pazz,

Here is a new pull request to workaround an annoying bug with powerpoint/excel attachments, even if not directly related to alot.

libmagic < 5.12 has a bug on microsoft office files: powerpoint/excel (and also visio I think) are incorrectly detected as application/msword (see http://bugs.gw.com/view.php?id=179 and http://bugs.gw.com/view.php?id=186).

As a result, depending on the mail client, recipients may have difficulties to directly open powerpoint attached because the mailer tries to open them with word.

Of course, the best solution is to use a more recent version of libmagic but the 5.11 is still present in a lot of distribution so I think this workaround might be useful.

The workaround is simple: if "application/msword" is detected, fallbacks on using python mimtypes modules which guess format based on filename. 

Yann