[PushHost] Use GUPnP Mime Type if available

updated based on V1 review comments.
----------
when hosting a file, we now use the mime type reported by GUPnP-DLNA
when available, and we fallback to the g_content_type_guess one if it
is not available.

Fix issue #120
https://github.com/01org/dleyna-renderer/issues/120

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>