Pasted images are broken

Summary

Images added from clipboard appear broken after publishing / saving the page.

Steps to Reproduce

1. Edit a page in Confluence.
2. Copy an image to clipboard using Mac OS screenshot feature. 
3. Paste an image using Cmd+V.
4. Observe an image added to the page.
5. Publish changes.

Expected Results

Pasted images should be saved correctly.

Actual Results

Pasted images always show as broken.

Notes

Browser: Safari

Instance: Pug, Fabric