bmcweb: Resolve issue with MIME type on redfish

The Redfish UI isn't presented properly with the latest security changes
because the MIME type wasn't explicitly set.  This resolves it, by
adding an MIME type to redfish.

Change-Id: Ic1b0c0bd6fb2b8b6b4d9393127e3e606ad7cb655
Signed-off-by: Ed Tanous <ed.tanous@intel.com>