OBJ written next to image

You should be able to add images by clicking on the block inserter icon on the top left of the text editor as outlined here. As an alternative, you can add an issue on the github source code repository of the plugin (https://github.com/wrCisco/simple-ebook-viewer-for-wp), where you can attach arbitrary file types, or you can find my contact info on my portfolio site (https://portfolio3d.francescomartini.it/).