The Select from page attachments option enables you to choose any existing diagram attached to the page with the .drawio extension, insert it into the page, and open it for editing.
When you click the Attachments button, a list of page attachments with the .drawio extension will be displayed:
Selecting the diagram will open it 
Select a diagram from the list above, and it will be opened in the editor.:
When Once you have finished working with the diagram, click the Save Save & Exit button to save the changes and exit editor or the editor. Alternatively, you can click Exit to discard the any changes made (if any) and leave.
After thatAfterwards, you 'll will see the a preview of the selected diagram.