I was recently trying to upload a newly built plugin to deploy on the Jenkins master in office, using the Update/Manage Plugins Advanced tab ($JENKINS_URL/pluginManager/advanced), and it failed with HTTP-502 (Bad gateway) reported by the reverse proxy (apache).

Looking into server logs, I saw a stacktrace topped by
Caused by: javax.servlet.ServletException: org.apache.commons.fileupload.FileUploadBase$IOFileUploadException: Processing of multipart/form-data request failed. java.util.concurrent.TimeoutException: Idle timeout expired: 5000/5000 ms

which seems to practically translate to "Jenkins does not accept big uploads over slow connections" (transfers taking over 5sec)?