Issue Summary

Attachment previews often fail to load with "Preview unavailable" errors within "Include page" macros.

Steps to Reproduce

This does not occur consistently, and may take some time to reproduce.

1. Create a page and add content. (Page "a")
2. Upload an attachment to the page
3. Create a second page and use the "Include page" macro to include the content from the page created in step 1 (Page "b")
4. Publish and load the page

Expected Results

Attachments hosted in page "a" preview as expected when loading pages "a" and "b".

Actual Results

Attachments hosted in page "a" preview as expected when loading pages "a". However, when loading page "b" these will intermittently fail to load the preview with a "Preview unavailable" error.

Workaround

This behaviour is intermittent. Hence, reloading the page may correct the symptom.