You can find this Parameter under Manage-(Core Admin/Administrator)Parameter-Search for this parameter under (Parameter Name or Value) and choose Selecting Partition as System and finally click on search button you will get -System.Messaging.Channels.File.FileUploadLimitInMB- Parameter with current value
This parameter is available for both Upstream and downstream
Since you need this parameter in upstream please check the same under MANAGE-ADMINISTRATOR and look for current value and if needed you need alter it and do system restart.