I found this Serverfault link <https://serverfault.com/questions/511789/nginx-client-request-body-is-buffered-to-a-temporary-file#answer-511790> that confirms that this warning is actually an Nginx message which is sent to indicate that the size of your uploaded file was larger than the in-memory buffer reserved for uploads.
About using the Nginx.config file provided on the link you shared, the link shared is for "a package that provides a Docker image running Jenkins behind NGINX <https://github.com/google/appengine-jenkins>". It is unclear how it directly applies to your setup. Can you provide more information about your application in the AppEngine Flex environment? -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to google-appengine+unsubscr...@googlegroups.com. To post to this group, send email to google-appengine@googlegroups.com. Visit this group at https://groups.google.com/group/google-appengine. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/073d93f5-c4b9-44f8-8135-f0cba7fa66fb%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
