Some block types as images or videos have to be added differently. For such types, you can either paste the source URL at any position (currently only images) or you download the file beforehand and drag and drop in here. You can always drag and drop files here and see if something happens. The supported file types will appear at the blue line in the app. The app supports also embed content from other platforms like YouTube (and more). For such cases just paste the URL and you will see something happen.