When you upload your eBook file at the “Add eBook Content” step, your file is immediately converted to a Kobo formatted ePub. After conversion is complete, the page will appear as follows, with a new link that says “Download and preview your ePub”.
To preview your ePub file, select the “Download and preview this eBook” link, and open your file in an ePub viewer. If you do not have an ePub viewer on your computer, you will need to download one in advance of opening the file. Note that there are many free ePub viewers available online, for example, Adobe Editions and Calibre are both free ePub viewers.