Catch ActionController::UnknownFormat and return HTTP 406 (#7621)

An error like that should not appear in production error log.

app/controllers/application_controller.rb

@@ -20,6 +20,7 @@ class ApplicationController < ActionController::Base
   rescue_from ActionController::RoutingError, with: :not_found
   rescue_from ActiveRecord::RecordNotFound, with: :not_found
   rescue_from ActionController::InvalidAuthenticityToken, with: :unprocessable_entity
+  rescue_from ActionController::UnknownFormat, with: :not_acceptable
   rescue_from Mastodon::NotPermittedError, with: :forbidden
 
   before_action :store_current_location, except: :raise_not_found, unless: :devise_controller?
@@ -73,6 +74,10 @@ class ApplicationController < ActionController::Base
     respond_with_error(422)
   end
 
+  def not_acceptable
+    respond_with_error(406)
+  end
+
   def single_user_mode?
     @single_user_mode ||= Rails.configuration.x.single_user_mode && Account.exists?
   end