Go this this page on your Kobo ereader and you see a unique key. Enter it in this form and upload an ebook and it will appear as a download link on the ereader.
If you send an EPUB file and tick the Kepubify checkbox, it will be converted into a Kobo EPUB using Kepubify. If you send a MOBI file to a Kobo, it will not be converted.
Your ebook will be stored on the server as long as your Kobo is viewing the unique key and is connected to wifi. It will be deleted irrevocably when the key expires about 30 seconds after you close the browser, generate a new key or disable wifi on your ereader.
By using this tool you agree that the ebook you upload is processed on the server and stored for a short time.