You can't upload any file from your machine to your "Rainbow-Link"-server directly. Since June 22, 2018, we provide an upload-server for you free of charge by using our hosting service, "もくれんのレンタルサーバー".
Your "Rainbow-Link"-server is in our data center located in Japan, but, the upload-server isn't in our data center. The upload-server for you is provided by another company. We provide our hosting service as a reseller of IFastNet LTD ( located in UK ).
Of course, you can use your own upload-server.