api: Do not guess based on name extension

Same issue as [0].

If a name ends in .json for example kube.json,
the API fails to process the request.

Create CT with name kube.json and try to do
openstack coe cluster template show kube.json

story: 1643995
task: 39862

[0] I789ecfeac9b64a9c4105a20619f7bf5dfc133189

Change-Id: I0db38880de0727aeed0290fd7f36002f17c0b8f2
Signed-off-by: Spyros Trigazis <strigazi@gmail.com>
(cherry picked from commit 07ba257d76)

magnum/api/app.py

@@ -43,6 +43,7 @@ def setup_app(config=None):
         app_conf.pop('root'),
         logging=getattr(config, 'logging', {}),
         wrap_app=middleware.ParsableErrorMiddleware,
+        guess_content_type_from_ext=False,
         **app_conf
     )