FIX: do not log if an invalid mime type is passed to app

Previously our custom exception handler was unable to handle situations
where an invalid mime type was sent, resulting in a warning log

This ensures we pretend a request is HTML for the purpose of rendering
the error page if an invalid mime type from a scanner is shipped to the app

spec/components/middleware/discourse_public_exceptions_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe Middleware::DiscoursePublicExceptions do
+  before do
+    @orig_logger = Rails.logger
+    Rails.logger = @fake_logger = FakeLogger.new
+  end
+
+  after do
+    Rails.logger = @orig_logger
+  end
+
+  def env(opts = {})
+    {
+      "HTTP_HOST" => "http://test.com",
+      "REQUEST_URI" => "/path?bla=1",
+      "REQUEST_METHOD" => "GET",
+      "rack.input" => ""
+    }.merge(opts)
+  end
+
+  it "should not log for invalid mime type requests" do
+    ex = Middleware::DiscoursePublicExceptions.new("/test")
+
+    ex.call(env(
+      "HTTP_ACCEPT" => "../broken../",
+      "action_dispatch.exception" => ActionController::RoutingError.new("abc")
+    ))
+
+    expect(@fake_logger.warnings.length).to eq(0)
+  end
+
+end