Select from attachments option allows to select any existing diagram with the .drawio extension, insert it into the page and, if necessary, update it using the editor.
After the Select from page attachments button click, the list of page’s attachments with the particular extension will be shown.
Clicking on the list item opens a diagram editor setting the selected diagram as a source.
When you have finished working with the diagram, click the Save & Exit button to save the changes and exit editor or Exit to discard the changes made (if any) and leave.
After that, you'll see the preview of the selected diagram.
