You can upload it from your computer or mobile device by clicking on the "Files" menu item in "New" menu:
The files you selected will be uploaded to the server and encrypted if the notebook is encrypted as well. They will be added as a subnotes, so you can easily manage them.
If you upload files to a note, their links will be appended to the note text. These links are id-based, therefore you can move files elsewhere within the current notebooks without losing their links.