Do not add mimetype to url as a query parameter

Bug: 11654127

Adding mimetype to url as a query parameter is a bad practice as
it requires us to inject app specific behavior to webview. There
also seems no reason to add this, so removing.

Change-Id: Ib2e5198981f2d37128ab545a33b83e8109bf3eef
(cherry picked from commit dd44be0b2e29ee043ac8c25dba34f11b9c018c46)