Merge branch 'monkey-patch-comment' into 'master'

Explain why we have this monkey-patch

See merge request !8329

config/initializers/workhorse_multipart.rb

@@ -2,6 +2,12 @@ Rails.application.configure do |config|
   config.middleware.use(Gitlab::Middleware::Multipart)
 end
 
+# The Gitlab::Middleware::Multipart middleware inserts instances of our
+# own ::UploadedFile class in the Rack env of requests. These instances
+# will be blocked by the 'strong parameters' feature of ActionController
+# unless we somehow whitelist them. At the moment it seems the only way
+# to do that is by monkey-patching.
+#
 module Gitlab
   module StrongParameterScalars
     GITLAB_PERMITTED_SCALAR_TYPES = [::UploadedFile]