Fix failing test_proxy_localhost_test_server
Fix failing test_proxy_localhost_test_server

The test server was using the content-type 'image/jpg'. The correct
mime type for JPEG images and the mime type defined in mime-types.json
is 'image/jpeg'.

atmos/camo,mozilla/mozillawebmakerproxy.net,Course-Master/camo,digideskio/camo,Course-Master/camo,espadrine/camo,missive/camo,grosser/camo