When you select the Open from Device option while adding a 
Simple Diagrams macro, you can choose any diagram with the .drawio extension stored on your device and update it using the editor. The selected diagram will be saved in the page’s attachments.
Clicking the Open from Device button will open the file manager.
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Note: only files with the .drawio extension can be selected, and you can choose only one file at a time. |
Once you select the diagram, it will open in the editor:
Once you've finished working on the diagram, click the Save & Exit button to save your changes and exit the editor. Alternatively, click Exit to discard any changes (if made) and leave.
Afterward, you'll see a preview of the uploaded diagram.