Add comments about always setting INFER_EXTENSION.

INFER_EXTENSION must always be set in order to provide right mimetypes for
some files.
For example mimetype for css files must be 'text/css' and not 'text/plain' as
returned by libmagic. If mimetype is not correct, css files of HTML5 webapps
will be not correctly interpreted resulting in a strange rendering in browser
like Chrome.

Change-Id: I7f0d9f97ede19308c87256f7850f5c51135827f3
Signed-off-by: Sebastien Douheret <sebastien.douheret@iot.bzh>